Aptamer-field-effect transistors overcome Debye length limitations for small-molecule sensing

A label-free fluorescence aptamer sensor for point-of-care serotonin detection

Serotonin, a pivotal neurotransmitter regulating various physiological functions, plays a crucial role in disease diagnosis, necessitating precise monitoring of its levels in biological fluids for accurate assessment. Aptamers, known for their high specificity and affinity, have emerged as innovative molecular probes for serotonin analysis. However, existing serotonin aptamer sensing platforms exhibit limitations in terms of portability and rapid detection capabilities. In this study, we introduce a novel, portable, label-free serotonin aptamer sensor utilizing a dye replacement strategy, achieving a short sample-to-result turnaround time and convenient signal readout through a smartphone. The performance of this aptamer sensor was thoroughly assessed across diverse physiological media, demonstrating robust stability in buffer, urine, and serum. Importantly, the detection limit was in the nanomolar range, emphasizing its suitability for the rapid, sensitive, and user-friendly detection of serotonin. This research pioneers an approach for the development of a point-of-care testing (POCT) system for serotonin with practical implications, particularly in resource-limited settings.

NAGase sensing in 3% milk: FET-based specific and label-free sensing in ultra-small samples of high ionic strength and high concentration of non-specific proteins

Biosensing with biological field-effect transistors (bioFETs) is a promising technology toward specific, label-free, and multiplexed sensing in ultra-small samples. The current study employs the field-effect meta-nano-channel biosensor (MNC biosensor) for the detection of the enzyme N-acetyl-beta-D-glucosaminidase (NAGase), a biomarker for milk cow infections. The measurements are performed in a 0.5 μL drops of 3% commercial milk spiked with NAGase concentrations in the range of 30.3 aM-3.03 μM (Note that there is no background NAGase concentration in commercial milk). Specific and label-free sensing of NAGase is demonstrated with a limit-of-detection of 30.3 aM, a dynamic range of 11 orders of magnitude and with excellent linearity and sensitivity. Additional two important research outcomes are reported. First, the ionic strength of the examined milk is ∼120 mM which implies a bulk Debye screening length <1 nm. Conventionally, a 1 nm Debye length excludes the possibility of sensing with a recognition layer composed of surface bound anti-NAGase antibodies with a size of ∼10 nm. This apparent contradiction is removed considering the ample literature reporting antibody adsorption in a predominantly surface tilted configuration (side-on, flat-on, etc.). Secondly, milk contains a non-specific background protein concentration of 33 mg/ml, in addition to considerable amounts of micron-size heterogeneous fat structures. The reported sensing was performed without the customarily exercised surface blocking and without washing of the non-specific signal. This suggests that the role of non-specific adsorption to the BioFET sensing signal needs to be further evaluated. Control measurements are reported.

Construction of AlGaN/GaN high-electron-mobility transistor-based biosensor for ultrasensitive detection of SARS-CoV-2 spike proteins and virions

The COVID-19 pandemic has highlighted the need for rapid and sensitive detection of SARS-CoV-2. Here, we report an ultrasensitive SARS-CoV-2 immunosensor by integration of an AlGaN/GaN high-electron-mobility transistor (HEMT) and anti-SARS-CoV-2 spike protein antibody. The AlGaN/GaN HEMT immunosensor has demonstrated the capability to detect SARS-CoV-2 spike proteins at an impressively low concentration of 10-22 M. The sensor was also applied to pseudoviruses and SARS-CoV-2 ΔN virions that display the Spike proteins with a single virion particle sensitivity. These features validate the potential of AlGaN/GaN HEMT biosensors for point of care tests targeting SARS-CoV-2. This research not only provides the first HEMT biosensing platform for ultrasensitive and label-free detection of SARS-CoV-2.

Cost-effective evaluation of Aptamer candidates in SELEX-based Aptamer isolation

Aptamers are short, single-stranded nucleic acids with high affinity and specificity for various targets, making them valuable in diagnostics and therapeutics. Their isolation traditionally involves a time-consuming and costly process called SELEX. While SELEX methods have evolved to improve binding and amplification, the crucial step of aptamer identification from sequencing data remains expensive and often overlooked. Common identification methods require modification of aptamer candidates with labels like biotin or fluorescent dyes, which becomes costly and cumbersome for high-throughput sequencing data. This paper presents an efficient and cost-effective approach to streamline aptamer identification. It employs asymmetric polymerase chain reaction (PCR) to generate modified single-stranded DNA copies of aptamer candidates, simplifying the modification process. By using excess modified forward primers and limited reverse primers, this method reduces costs since only unmodified candidates need to be synthesized initially. The approach was demonstrated with an IgE protein aptamer and successfully applied to identify aptamers from a pool of 12 candidates against a monoclonal antibody. The validity of the results was further confirmed through the direct synthesis of fluorophore-conjugated aptamer candidates, yielding consistent outcomes while reducing the cost by threefold. This approach addresses a critical bottleneck in aptamer discovery by significantly reducing the time and cost associated with aptamer identification, facilitating aptamer-based research and making aptamers more accessible for various applications in diagnostics and therapeutics.

Self-Assembled Monolayer Transporters Enable Reagentless Analysis of Small Molecule Analytes

The detection of small molecules beyond glucose remains an ongoing challenge in the field of biomolecular sensing owing to their small size, diverse structures, and lack of alternative non-enzymatic sensing methods. Here, we present a new reagentless electrochemical approach for small molecule detection that involves directed movement of electroactive analytes through a self-assembled monolayer to an electrode surface. Using this method, we demonstrate detection of several physiologically relevant small molecules as well as the capacity for the system to operate in several biological fluids. We anticipate that this mechanism will further improve our capacity for small molecule measurement and provide a new generalizable monolayer-based technique for electrochemical assessment of various electroactive analytes.

Nucleic acid-based wearable and implantable electrochemical sensors

The rapid advancements in nucleic acid-based electrochemical sensors for implantable and wearable applications have marked a significant leap forward in the domain of personal healthcare over the last decade. This technology promises to revolutionize personalized healthcare by facilitating the early diagnosis of diseases, monitoring of disease progression, and tailoring of individual treatment plans. This review navigates through the latest developments in this field, focusing on the strategies for nucleic acid sensing that enable real-time and continuous biomarker analysis directly in various biofluids, such as blood, interstitial fluid, sweat, and saliva. The review delves into various nucleic acid sensing strategies, emphasizing the innovative designs of biorecognition elements and signal transduction mechanisms that enable implantable and wearable applications. Special perspective is given to enhance nucleic acid-based sensor selectivity and sensitivity, which are crucial for the accurate detection of low-level biomarkers. The integration of such sensors into implantable and wearable platforms, including microneedle arrays and flexible electronic systems, actualizes their use in on-body devices for health monitoring. We also tackle the technical challenges encountered in the development of these sensors, such as ensuring long-term stability, managing the complexity of biofluid dynamics, and fulfilling the need for real-time, continuous, and reagentless detection. In conclusion, the review highlights the importance of these sensors in the future of medical engineering, offering insights into design considerations and future research directions to overcome existing limitations and fully realize the potential of nucleic acid-based electrochemical sensors for healthcare applications.

Modular Fabrication of Microfluidic Graphene FET for Nucleic Acids Biosensing

Graphene field-effect transistors (GFETs) are widely used in biosensing due to their excellent properties in biomolecular signal amplification, exhibiting great potential for high-sensitivity and point-of-care testing in clinical diagnosis. However, difficulties in complicated fabrication steps are the main limitations for the further studies and applications of GFETs. In this study, a modular fabrication technique is introduced to construct microfluidic GFET biosensors within 3 independent steps. The low-melting metal electrodes and intricate flow channels are incorporated to maintain the structural integrity of graphene and facilitate subsequent sensing operations. The as-fabricated GFET biosensor demonstrates excellent long-term stability, and performs effectively in various ion environments. It also exhibits high sensitivity and selectivity for detecting single-stranded nucleic acids at a 10 fm concentration. Furthermore, when combined with the CRISPR/Cas12a system, it facilitates amplification-free and rapid detection of nucleic acids at a concentration of 1 fm. Thus, it is believed that this modular-fabricated microfluidic GFET may shed light on further development of FET-based biosensors in various applications.

Wearable and Multiplexed Biosensors based on Oxide Field-Effect Transistors

Wearable sensors designed for continuous, non-invasive monitoring of physicochemical signals are important for portable healthcare. Oxide field-effect transistor (FET)-type biosensors provide high sensitivity and scalability. However, they face challenges in mechanical flexibility, multiplexed sensing of different modules, and the absence of integrated on-site signal processing and wireless transmission functionalities for wearable sensing. In this work, a fully integrated wearable oxide FET-based biosensor array is developed to facilitate the multiplexed and simultaneous measurement of ion concentrations (H+, Na+, K+) and temperature. The FET-sensor array is achieved by utilizing a solution-processed ultrathin (≈6 nm thick) In2O3 active channel layer, exhibiting high compatibility with standard semiconductor technology, good mechanical flexibility, high uniformity, and low operational voltage of 0.005 V. This work provides an effective method to enable oxide FET-based biosensors for the fusion of multiplexed physicochemical information and wearable health monitoring applications.

Single-Molecule Electrical Profiling of Peptides and Proteins

In recent decades, there has been a significant increase in the application of single-molecule electrical analysis platforms in studying proteins and peptides. These advanced analysis methods have the potential for deep investigation of enzymatic working mechanisms and accurate monitoring of dynamic changes in protein configurations, which are often challenging to achieve in ensemble measurements. In this work, the prominent research progress in peptide and protein-related studies are surveyed using electronic devices with single-molecule/single-event sensitivity, including single-molecule junctions, single-molecule field-effect transistors, and nanopores. In particular, the successful commercial application of nanopores in DNA sequencing has made it one of the most promising techniques in protein sequencing at the single-molecule level. From single peptides to protein complexes, the correlation between their electrical characteristics, structures, and biological functions is gradually being established. This enables to distinguish different molecular configurations of these biomacromolecules through real-time electrical monitoring of their life activities, significantly improving the understanding of the mechanisms underlying various life processes.

Universal Covalent Grafting Strategy of an Aptamer on a Carbon Fiber Microelectrode for Selective Determination of Dopamine In Vivo

The chemical information on brain science provided by electrochemical sensors is critical for understanding brain chemistry during physiological and pathological processes. A major challenge is the selectivity of electrochemical sensors in vivo. This work developed a universal covalent grafting strategy of an aptamer on a carbon fiber microelectrode (CFE) for selective determination of dopamine in vivo. The universal strategy was proposed by oxidizing poly(tannic acid) (pTA) to form an oxidized state (pTAox) and then coupling a nucleophilic sulfhydryl molecule of the dopamine-binding mercapto-aptamer with the o-quinone moiety of pTAox based on click chemistry for the interfacial functionalization of the CFE surface. It was found that the universal strategy proposed could efficiently graft the aptamer on a glassy carbon electrode, which was verified by using electroactive 6-(ferrocenyl) hexanethiol as a redox reporter. The amperometric method using a fabricated aptasensor for the determination of dopamine was developed. The linear range of the aptasensor for the determination of dopamine was 0.2-20 μM with a sensitivity of 0.09 nA/μM and a limit of detection of 88 nM (S/N = 3). The developed method has high selectivity originating from the specific recognition of the aptamer in concert with the cation-selective action of pTA and could be easily applicable to probe dopamine dynamics in the brain. Furthermore, complex vesicle fusion modes were first observed at the animal level. This work demonstrated that the covalently grafted immobilization strategy proposed is promising and could be extended to the in vivo analysis of other neurochemicals.

Whole blood multiplex measurements using electrochemical aptamer-based biosensors

Simultaneous measurements of various molecules ("multiplex") using electrochemical biosensors typically require multiple electrode implementations, which for neonates, hemophiliacs, etc. is problematic. Here, we introduce the oxazine ATTO 700 into electrochemical aptamer-based biosensors to achieve "true" multiplex, continuous and real-time measurements of two different molecules in undiluted whole blood using a single electrode.

An Ion-Mediated Spiking Chemical Neuron based on Mott Memristor

Artificial spiking neurons capable of interpreting ionic information into electrical spikes are critical to mimic biological signaling systems. Mott memristors are attractive for constructing artificial spiking neurons due to their simple structure, low energy consumption, and rich neural dynamics. However, challenges remain in achieving ion-mediated spiking and biohybrid-interfacing in Mott neurons. Here, a biomimetic spiking chemical neuron (SCN) utilizing an NbOx Mott memristor and oxide field-effect transistor-type chemical sensor is introduced. The SCN exhibits both excitation and inhibition spiking behaviors toward ionic concentrations akin to biological neural systems. It demonstrates spiking responses across physiological and pathological Na+ concentrations (1-200 × 10-3 m). The Na+-mediated SCN enables both frequency encoding and time-to-first-spike coding schemes, illustrating the rich neural dynamics of Mott neuron. In addition, the SCN interfaced with L929 cells facilitates real-time modulation of ion-mediated spiking under both normal and salty cellular microenvironments.

Unexpected enrichment of DNA aptamers for Zn2+ ions from an insulin selection

We serendipitously discovered Zn2+-binding DNA aptamers when selecting insulin aptamers. The Zn-1 aptamer binds to Zn2+ with a dissociation constant (Kd) of ∼1 μM, and has 450-fold higher selectivity for Zn2+ over Cd2+. A strand-displacement based fluorescent sensor achieved a limit of detection of 0.2 μM Zn2+.

Merging DNA Repair with Bioorthogonal Conjugation Enables Accessible and Versatile Asymmetric DNA Catalysis

Optimizing catalysts through high-throughput screening for asymmetric catalysis is challenging due to the difficulty associated with assembling a library of catalyst analogues in a timely fashion. Here, we repurpose DNA excision repair and integrate it with bioorthogonal conjugation to construct a diverse array of DNA hybrid catalysts for highly accessible and high-throughput asymmetric DNA catalysis, enabling a dramatically expedited catalyst optimization process, superior reactivity and selectivity, as well as the first atroposelective DNA catalysis. The bioorthogonality of this conjugation strategy ensures exceptional tolerance toward diverse functional groups, thereby facilitating the facile construction of 44 DNA hybrid catalysts bearing various unprotected functional groups. This unique feature holds the potential to enable catalytic modalities in asymmetric DNA catalysis that were previously deemed unattainable.

Kinetic ITC of DNA Aptamers Binding for Small Molecules and Implications for Binding Assays and Biosensors

The determination of kon and koff values through kinetic analysis is crucial for understanding the intricacies of aptamer-target binding interactions. By employing kinetic ITC, we systematically analyzed a range of ITC data of various aptamers. Upon plotting their kon and koff values as a function of their Kd values, a notable trend emerged. Across a range of Kd values spanning from 28 nM to 864 μM, the kon value decreased from 2×105 M-1 s-1 to 96 M-1 s-1, whereas the koff value increased from 1.03×10-3 s-1 to 0.012 s-1. Thus, both kon and koff contributed to the change of Kd in the same direction, although the range of kon change was larger. Since experiments are often run at close to the Kd value, this concentration effect also played an important role in the observed binding kinetics. The effect of these kinetic parameters on two common sensing mechanisms, including aptamer beacons and strand-displacement assays, are discussed. This work has provided the kinetic values of small molecule binding aptamers and offered insights into aptamer-based biosensors.

Aptamer-Based Potentiometric Sensor Enables Highly Selective and Neurocompatible Neurochemical Sensing in Rat Brain

Selective and nondisruptive in vivo neurochemical monitoring within the central nervous system has long been a challenging endeavor. We introduce a new sensing approach that integrates neurocompatible galvanic redox potentiometry (GRP) with customizable phosphorothioate aptamers to specifically probe dopamine (DA) dynamics in live rat brains. The aptamer-functionalized GRP (aptGRP) sensor demonstrates nanomolar sensitivity and over a 10-fold selectivity for DA, even amidst physiological levels of major interfering species. Notably, conventional sensors without the aptamer modification exhibit negligible reactivity to DA concentrations exceeding 20 μM. Critically, the aptGRP sensor operates without altering neuronal activity, thereby permitting real-time, concurrent recordings of both DA flux and electrical signaling in vivo. This breakthrough establishes aptGRP as a viable and promising framework for the development of high-fidelity sensors, offering novel insights into neurotransmission dynamics in a live setting.

Wearable Aptalyzer Integrates Microneedle and Electrochemical Sensing for In Vivo Monitoring of Glucose and Lactate in Live Animals

Continuous monitoring of clinically relevant biomarkers within the interstitial fluid (ISF) using microneedle (MN)-based assays, has the potential to transform healthcare. This study introduces the Wearable Aptalyzer, an integrated system fabricated by combining biocompatible hydrogel MN arrays for ISF extraction with an electrochemical aptamer-based biosensor for in situ monitoring of blood analytes. The use of aptamers enables continuous monitoring of a wide range of analytes, beyond what is possible with enzymatic monitoring. The Wearable Aptalyzer is used for real-time and multiplexed monitoring of glucose and lactate in ISF. Validation experiments using live mice and rat models of type 1 diabetes demonstrate strong correlation between the measurements collected from the Wearable Aptalyzer in ISF and those obtained from gold-standard techniques for blood glucose and lactate, for each analyte alone and in combination. The Wearable Aptalyzer effectively addresses the limitations inherent in enzymatic detection methods as well as solid MN biosensors and the need for reliable and multiplexed bioanalytical monitoring in vivo.

A portable transistor immunosensor for fast identification of porcine epidemic diarrhea virus

Widespread distribution of porcine epidemic diarrhea virus (PEDV) has led to catastrophic losses to the global pig farming industry. As a result, there is an urgent need for rapid, sensitive and accurate tests for PEDV to enable timely and effective interventions. In the present study, we develop and validate a floating gate carbon nanotubes field-effect transistor (FG CNT-FET)-based portable immunosensor for rapid identification of PEDV in a sensitive and accurate manner. To improve the affinity, a unique PEDV spike protein-specific monoclonal antibody is prepared by purification, and subsequently modified on FG CNT-FET sensor to recognize PEDV. The developed FET biosensor enables highly sensitive detection (LoD: 8.1 fg/mL and 100.14 TCID50/mL for recombinant spike proteins and PEDV, respectively), as well as satisfactory specificity. Notably, an integrated portable platform consisting of a pluggable FG CNT-FET chip and a portable device can discriminate PEDV positive from negative samples and even identify PEDV and porcine deltacoronavirus within 1 min with 100% accuracy. The portable sensing platform offers the capability to quickly, sensitively and accurately identify PEDV, which further points to a possibility of point of care (POC) applications of large-scale surveillance in pig breeding facilities.

Noncompetitive Determination of Small Analytes by Sandwich-Type Lateral Flow Assay Based on an Aptamer Kissing Complex

Here, we present the proof-of-concept of a lateral flow assay (LFA) that is capable of detecting small-molecule targets in a noncompetitive manner by deploying a sandwich-type format based on the aptamer kissing complex (AKC) strategy. A fluorescently labeled hairpin aptamer served as the signaling agent, while a specific RNA hairpin grafted onto the strip served as the capture element. The hairpin aptamer switched from an unfolded to a folded form in the presence of the target, resulting in kissing interactions between the loops of the reporter and the capture agents. This design triggered a target-dependent fluorescent signal at the test line. The AKC-based LFA was developed for the detection of adenosine, achieving a detection limit in the micromolar range. The assay revealed the presence of the same analyte in urine. The method also proved effective with another small molecule (theophylline). We believe that the AKC-based LFA approach could overcome many of the shortcomings associated with conventional signal-off methods and competitive processes.

Turn-on fluorescent aptasensing for the determination of serotonin via target-induced knot displacement at corona

A turn-on fluorescence aptasensing approach for the highly sensitive and selective determination of 5-HT was proposed via target-induced knot displacement. 5-HT can be determined in a range from 0.5 nM to 100 nM with a limit of detection as low as 0.1 nM and a low dissociation constant of 2.3 nM.

Aptasensor-encapsulating semi-permeable proteinosomes for direct target detection in non-treated biofluids

Detecting biomarkers in biofluids directly without sample treatments makes molecular diagnostics faster and more efficient. Aptasensors, the nucleic acid-based molecular biosensors, can detect a wide range of target molecules, but their susceptibility to degradation and aggregation by nucleases and charged proteins, respectively, limits their direct use in clinical samples. In this work, we demonstrate that when aptasensors are encapsulated in proteinosomes, the protein-based liposome mimics, clinically important small molecules can be sensitively and selectively detected in non-treated specimens, such as 100 % unpurified serum. As serum albumin is used to form the membrane, the nanomeshed proteinosomes become semi-permeable and antifouling, which enables exclusive admission of small molecules while blocking unwanted large proteins. Consequently, the enclosed aptasensors can maintain close-to-optimal performance for target binding, and nucleolytic degradation and electrostatic aggregation are effectively suppressed. Three different structure-switching aptamers specific for estradiol, dopamine, and cocaine, respectively, are demonstrated to fully conserve their high affinities and specificities inside the microcapsules. The shielding effect of proteinosomes is indeed exceptional; the enclosed DNA aptasensors remain completely intact over 18 h in serum and even in an extremely concentrated DNase solution (1 mg/ml, ∼300,000× the serum level). Moreover, the proteinosome-mediated compartmentalization enables independent operation of multiple aptasensors in the same mixture. Hence, simultaneous real-time sensing of two different targets is demonstrated with different operation modes, 'recording' target appearance and 'reporting' target concentration changes. This work is the first demonstration of small-molecule-specific aptasensors operating with optimal performance in serum environments and will find promising applications in molecular diagnostics.

In vitro selection of a trans aptamer complex for target-responsive fluorescence activation

BACKGROUND

Most biological molecular complexes consist of multiple functional domains, yet rationally constructing such multifunctional complexes is challenging. Aptamers, the nucleic acid-based functional molecules, can perform multiple tasks including target recognition, conformational changes, and enzymatic activities, while being chemically synthesizable and tunable, and thus provide a basis for engineering enhanced functionalities through combination of multiple units. However, the conventional approach of simply combining aptamer units in a serial manner is susceptible to undesired crosstalk or interference between the aptamer units and to false interactions with non-target molecules; besides, the approach would require additional mechanisms to separate the units if they are desired to function independently. It is clearly a challenge to develop multi-aptamer complexes that preserve independent functions of each unit while avoiding undesired interference and non-specific interactions.

RESULTS

By directly in vitro selecting a 'trans' aptamer complex, we demonstrate that one aptamer unit ('utility module') can remain hidden or 'inactive' until a target analyte triggers the other unit ('sensing module') and separates the two aptamers. Since the operation of the utility module occurs free from the sensing module, unnecessary crosstalk between the two units can be avoided. Because the utility module is kept inactive until separated from the complex, non-specific interactions of the hidden module with noncognate targets can be naturally prevented. In our demonstration, the sensing module was selected to detect serotonin, a clinically important neurotransmitter, and the target-binding-induced structure-switching of the sensing module reveals and activates the utility module that turns on a fluorescence signal. The aptamer complex exhibited a moderately high affinity and an excellent specificity for serotonin with ∼16-fold discrimination against common neurotransmitter molecules, and displayed strong robustness to perturbations in the design, disallowing nonspecific reactions against various challenges.

SIGNIFICANCE

This work represents the first example of a trans aptamer complex that was in vitro selected de novo. The trans aptamer complex selected by our strategy does not require chemical modifications or immediate optimization processes to function, because the complex is directly selected to perform desired functions. This strategy should be applicable to a wide range of functional nucleic acid moieties, which will open up diverse applications in biosensing and molecular therapeutics.

Single-Step Functionalization Strategy of Graphene Microtransistor Array with Chemically Modified Aptamers for Biosensing Applications

Graphene solution-gated field-effect transistors (gSGFETs) offer high potential for chemical and biochemical sensing applications. Among the current trends to improve this technology, the functionalization processes are gaining relevance for its crucial impact on biosensing performance. Previous efforts are focused on simplifying the attachment procedure from standard multi-step to single-step strategies, but they still suffer from overreaction, and impurity issues and are limited to a particular ligand. Herein, a novel strategy for single-step immobilization of chemically modified aptamers with fluorenylmethyl and acridine moieties, based on a straightforward synthetic route to overcome the aforementioned limitations is presented. This approach is benchmarked versus a standard multi-step strategy using thrombin as detection model. In order to assess the reliability of the functionalization strategies 48-gSGFETs arrays are employed to acquire large datasets with multiple replicas. Graphene surface characterization demonstrates robust and higher efficiency in the chemical coupling of the aptamers with the single-step strategy, while the electrical response evaluation validates the sensing capability, allowing to implement different alternatives for data analysis and reduce the sensing variability. In this work, a new tool capable of overcome the functionalization challenges of graphene surfaces is provided, paving the way toward the standardization of gSGFETs for biosensing purposes.

Carbon-nanotube field-effect transistors for resolving single-molecule aptamer-ligand binding kinetics

Small molecules such as neurotransmitters are critical for biochemical functions in living systems. While conventional ultraviolet-visible spectroscopy and mass spectrometry lack portability and are unsuitable for time-resolved measurements in situ, techniques such as amperometry and traditional field-effect detection require a large ensemble of molecules to reach detectable signal levels. Here we demonstrate the potential of carbon-nanotube-based single-molecule field-effect transistors (smFETs), which can detect the charge on a single molecule, as a new platform for recognizing and assaying small molecules. smFETs are formed by the covalent attachment of a probe molecule, in our case a DNA aptamer, to a carbon nanotube. Conformation changes on binding are manifest as discrete changes in the nanotube electrical conductance. By monitoring the kinetics of conformational changes in a binding aptamer, we show that smFETs can detect and quantify serotonin at the single-molecule level, providing unique insights into the dynamics of the aptamer-ligand system. In particular, we show the involvement of G-quadruplex formation and the disruption of the native hairpin structure in the conformational changes of the serotonin-aptamer complex. The smFET is a label-free approach to analysing molecular interactions at the single-molecule level with high temporal resolution, providing additional insights into complex biological processes.

Ultra-sensitive and rapid detection of Salmonella enterica and Staphylococcus aureus to single-cell level by aptamer-functionalized carbon nanotube field-effect transistor biosensors

Foodborne diseases caused by Salmonella enterica (S. enterica) and Staphylococcus aureus (S. aureus) significantly impact public health, underscoring the imperative for highly sensitive, rapid, and accurate detection technologies to ensure food safety and prevent human diseases. Nanomaterials hold great promise in the development of high-sensitivity transistor biosensors. In this work, field-effect transistor (FET) comprising high-purity carbon nanotubes (CNTs) were fabricated and modified with corresponding nucleic acid aptamers for the high-affinity and selective capture of S. enterica and S. aureus. The aptamer-functionalized CNT-FET biosensor demonstrated ultra-sensitive and rapid detection of these foodborne pathogens. Experimental results indicated that the biosensor could detect S. enterica at a limit of detection (LOD) as low as 1 CFU in PBS buffer, and S. aureus at an LOD of 1.2 CFUs, achieving single-cell level detection accuracy with exceptional specificity. The biosensor exhibited a rapid response time, completing single detections within 200 s. Even in the presence of interference from six complex food matrices, the biosensor maintained its ultra-sensitive (3.1 CFUs) and rapid response (within 200 s) characteristics for both pathogens. The developed aptamer-functionalized CNT-FET biosensor demonstrates a capability for low-cost, ultra-sensitive, label-free, and rapid detection of low-abundance S. enterica and S. aureus in both buffer solutions and complex environments. This innovation holds significant potential for applying this detection technology to on-site rapid testing scenarios, offering a promising solution to the pressing need for efficient and reliable pathogen monitoring in various settings.

Antibody Nanotweezer Constructing Bivalent Transistor-Biomolecule Interface with Spatial Tolerance

Establishing a multivalent interface between the biointerface of a living system and electronic device is vital to building intelligent bioelectronic systems. How to achieve multivalent binding with spatial tolerance at the nanoscale remains challenging. Here, we report an antibody nanotweezer that is a self-adaptive bivalent nanobody enabling strong and resilient binding between transistor and envelope proteins at biointerfaces. The antibody nanotweezer is constructed by a DNA framework, where the nanoscale patterning of nanobodies along with their local spatial adaptivity enables simultaneous recognition of target epitopes without binding stress. As such, effective binding affinity increases by 1 order of magnitude compared with monovalent antibody. The antibody nanotweezer operating on transistor offers enhanced signal transduction, which is implemented to detect clinical pathogens, showing ∼100% overall agreement with PCR results. This work provides a perspective of engineering multivalent interfaces between biosystems with solid-state devices, holding great potential for organoid intelligence on a chip.

Spatial imaging of glycoRNA in single cells with ARPLA

Little is known about the biological roles of glycosylated RNAs (glycoRNAs), a recently discovered class of glycosylated molecules, because of a lack of visualization methods. We report sialic acid aptamer and RNA in situ hybridization-mediated proximity ligation assay (ARPLA) to visualize glycoRNAs in single cells with high sensitivity and selectivity. The signal output of ARPLA occurs only when dual recognition of a glycan and an RNA triggers in situ ligation, followed by rolling circle amplification of a complementary DNA, which generates a fluorescent signal by binding fluorophore-labeled oligonucleotides. Using ARPLA, we detect spatial distributions of glycoRNAs on the cell surface and their colocalization with lipid rafts as well as the intracellular trafficking of glycoRNAs through SNARE protein-mediated secretory exocytosis. Studies in breast cell lines suggest that surface glycoRNA is inversely associated with tumor malignancy and metastasis. Investigation of the relationship between glycoRNAs and monocyte-endothelial cell interactions suggests that glycoRNAs may mediate cell-cell interactions during the immune response.

Electrochemical and biosensor techniques to monitor neurotransmitter changes with depression

Depression is a common mental illness. However, its current treatments, like selective serotonin reuptake inhibitors (SSRIs) and micro-dosing ketamine, are extremely variable between patients and not well understood. Three neurotransmitters: serotonin, histamine, and glutamate, have been proposed to be key mediators of depression. This review focuses on analytical methods to quantify these neurotransmitters to better understand neurological mechanisms of depression and how they are altered during treatment. To quantitatively measure serotonin and histamine, electrochemical techniques such as chronoamperometry and fast-scan cyclic voltammetry (FSCV) have been improved to study how specific molecular targets, like transporters and receptors, change with antidepressants and inflammation. Specifically, these studies show that different SSRIs have unique effects on serotonin reuptake and release. Histamine is normally elevated during stress, and a new inflammation hypothesis of depression links histamine and cytokine release. Electrochemical measurements revealed that stress increases histamine, decreases serotonin, and leads to changes in cytokines, like interleukin-6. Biosensors can also measure non-electroactive neurotransmitters, including glutamate and cytokines. In particular, new genetic sensors have shown how glutamate changes with chronic stress, as well as with ketamine treatment. These techniques have been used to characterize how ketamine changes glutamate and serotonin, and to understand how it is different from SSRIs. This review briefly outlines how these electrochemical techniques work, but primarily highlights how they have been used to understand the mechanisms of depression. Future studies should explore multiplexing techniques and personalized medicine using biomarkers in order to investigate multi-analyte changes to antidepressants.

In Silico Study on a Binding Mechanism of ssDNA Aptamers Targeting Glycosidic Bond-Containing Small Molecules

Aptamer-based detection targeting glycoconjugates has attracted significant attention for its remarkable potential in identifying structural changes in saccharides in different stages of various diseases. However, the challenges in screening aptamers for small carbohydrates or glycoconjugates, which contain highly flexible and diverse glycosidic bonds, have hindered their application and commercialization. In this study, we investigated the binding conformations between three glycosidic bond-containing small molecules (GlySMs; glucose, N-acetylneuraminic acid, and neomycin) and their corresponding aptamers in silico, and analyzed factors contributing to their binding affinities. Based on the findings, a novel binding mechanism was proposed, highlighting the central role of the stem structure of the aptamer in binding and recognizing GlySMs and the auxiliary role of the mismatched bases in the adjacent loop. Guided by this binding mechanism, an aptamer with a higher 6'-sialyllactose binding affinity was designed, achieving a KD value of 4.54 ± 0.64 μM in vitro through a single shear and one mutation. The binding mechanism offers crucial guidance for designing high-affinity aptamers, enhancing the virtual screening efficiency for GlySMs. This streamlined workflow filters out ineffective binding sites, accelerating aptamer development and providing novel insights into glycan-nucleic acid interactions.

Aptamer-Antibody Chimera Sensors for Sensitive, Rapid, and Reversible Molecular Detection in Complex Samples

The development of receptors suitable for the continuous detection of analytes in complex, interferent-rich samples remains challenging. Antibodies are highly sensitive but difficult to engineer in order to introduce signaling functionality, while aptamer switches are easy to construct but often yield only a modest target sensitivity. We present here a programmable antibody and DNA aptamer switch (PANDAS), which combines the desirable properties of both receptors by using a nucleic acid tether to link an analyte-specific antibody to an internal strand-displacement (ISD)-based aptamer switch that recognizes the same target through different epitopes. The antibody increases PANDAS analyte binding due to its high affinity, and the effective concentration between the two receptors further enhances two-epitope binding and fluorescent aptamer signaling. We developed a PANDAS sensor for the clotting protein thrombin and show that a tuned design achieves a greater than 300-fold enhanced sensitivity compared to that of using an aptamer alone. This design also exhibits reversible binding, enabling repeated measurements with a temporal resolution of ∼10 min, and retains excellent sensitivity even in interferent-rich samples. With future development, this PANDAS approach could enable the adaptation of existing protein-binding aptamers with modest affinity to sensors that deliver excellent sensitivity and minute-scale resolution in minimally prepared biological specimens.

Spiers Memorial Lecture: Challenges and prospects in organic photonics and electronics

While a substantial amount of research activity has been conducted in fields related to organic photonics and electronics, including the development of devices such as organic field-effect transistors, organic photovoltaics, and organic light-emitting diodes for applications encompassing organic thermoelectrics, organic batteries, excitonic organic materials for photochemical and optoelectronic applications, and organic thermoelectrics, this perspective review will primarily concentrate on the emerging and rapidly expanding domain of organic bioelectronics and neuromorphics. Here we present the most recent research findings on organic transistors capable of sensing biological biomarkers down at the single-molecule level (i.e., oncoproteins, genomes, etc.) for the early diagnosis of pathological states and to mimic biological synapses, paving the way to neuromorphic applications that surpass the limitations of the traditional von Neumann computing architecture. Both organic bioelectronics and neuromorphics exhibit several challenges but will revolutionize human life, considering the development of artificial synapses to counteract neurodegenerative disorders and the development of ultrasensitive biosensors for the early diagnosis of cancer to prevent its development. Moreover, organic bioelectronics for sensing applications have also triggered the development of several wearable, flexible and stretchable biodevices for continuous biomarker monitoring.

Polymeric integration of structure-switching aptamers on transistors for histamine sensing

Aptamers that undergo large conformational rearrangements at the surface of electrolyte-gated field-effect transistor (EG-FETs)-based biosensors can overcome the Debye length limitation in physiological high ionic strength environments. For the sensitive detection of small molecules, carbon nanotubes (CNTs) that approach the dimensions of analytes of interest are promising channel materials for EG-FETs. However, functionalization of CNTs with bioreceptors using frequently reported surface modification strategies (e.g., π-π stacking), requires highly pristine CNTs deposited through methods that are incompatible with low-cost fabrication methods and flexible substrates. In this work, we explore alternative non-covalent surface chemistry to functionalize CNTs with aptamers. We harnessed the adhesive properties of poly-D-lysine (PDL), to coat the surface of CNTs and then grafted histamine-specific DNA aptamers electrostatically in close proximity to the CNT semiconducting channel. The layer-by-layer assembly was monitored by complementary techniques such as X-ray photoelectron spectroscopy, optical waveguide lightmode spectroscopy, and fluorescence microscopy. Surface characterization confirmed histamine aptamer integration into PDL-coated CNTs and revealed ∼5-fold higher aptamer surface coverage when using CNT networks with high surface areas. Specific aptamers assembled on EG-CNTFETs enabled histamine detection in undiluted high ionic strength solutions in the concentration range of 10 nM to 100 μM. Sequence specificity was demonstrated via parallel measurements with control EG-CNTFETs functionalized with scrambled DNA. Histamine aptamer-modified EG-CNTFETs showed high selectivity vs. histidine, the closest structural analog and precursor to histamine. Taken together, these results implied that target-specific aptamer conformational changes on CNTs facilitate signal transduction, which was corroborated by circular dichroism spectroscopy. Our work suggests that layer-by-layer polymer chemistry enables integration of structure-switching aptamers into flexible EG-CNTFETs for small-molecule biosensing.

Aptamer-Functionalized Interface Nanopores Enable Amino Acid-Specific Peptide Detection

Single-molecule proteomics based on nanopore technology has made significant advances in recent years. However, to achieve nanopore sensing with single amino acid resolution, several bottlenecks must be tackled: controlling nanopore sizes with nanoscale precision and slowing molecular translocation events. Herein, we address these challenges by integrating amino acid-specific DNA aptamers into interface nanopores with dynamically tunable pore sizes. A phenylalanine aptamer was used as a proof-of-concept: aptamer recognition of phenylalanine moieties led to the retention of specific peptides, slowing translocation speeds. Importantly, while phenylalanine aptamers were isolated against the free amino acid, the aptamers were determined to recognize the combination of the benzyl or phenyl and the carbonyl group in the peptide backbone, enabling binding to specific phenylalanine-containing peptides. We decoupled specific binding between aptamers and phenylalanine-containing peptides from nonspecific interactions (e.g., electrostatics and hydrophobic interactions) using optical waveguide lightmode spectroscopy. Aptamer-modified interface nanopores differentiated peptides containing phenylalanine vs. control peptides with structurally similar amino acids (i.e., tyrosine and tryptophan). When the duration of aptamer-target interactions inside the nanopore were prolonged by lowering the applied voltage, discrete ionic current levels with repetitive motifs were observed. Such reoccurring signatures in the measured signal suggest that the proposed method has the possibility to resolve amino acid-specific aptamer recognition, a step toward single-molecule proteomics.

Local Electric Potential-Driven Nanofluidic Ion Transport for Ultrasensitive Biochemical Sensing

Nanofluidic biosensors have been widely used for detection of analytes based on the change of system resistance before and after target-probe interactions. However, their sensitivity is limited when system resistance barely changes toward low-concentration targets. Here, we proposed a strategy to address this issue by means of target-induced change of local membrane potential under relatively unchanged system resistance. The local membrane potential originated from the directional diffusion of photogenerated carriers across nanofluidic biosensors and gated photoinduced ionic current signal before and after target-probe interactions. The sensitivity of such biosensors for the detection of biomolecules such as circulating tumor DNA (ctDNA) and lysozyme exceeds that of applying a traditional strategy by more than 3 orders of magnitude under unchanged system resistance. Such biosensors can specifically detect the small molecule biomarker in the blood sample between prostate cancer patients and healthy humans. The key advantages of such nanofluidic biosensors are therefore complementary to traditional nanofluidic biosensors, with potential applications in a point-of-care analytical tool.

Exploring the Landscape of Aptamers: From Cross-Reactive to Selective to Specific, High-Affinity Receptors for Cocaine

We reported over 20 years ago MNS-4.1, the first DNA aptamer with a micromolar affinity for cocaine. MNS-4.1 is based on a structural motif that is very common in any random pool of oligonucleotides, and it is actually a nonspecific hydrophobic receptor with wide cross-reactivity with alkaloids and steroids. Despite such weaknesses preventing broad applications, this aptamer became widely used in proof-of-concept demonstrations of new formats of biosensors. We now report a series of progressively improved DNA aptamers recognizing cocaine, with the final optimized receptors having low nanomolar affinity and over a thousand-fold selectivity over the initial cross-reactants. In the process of optimization, we tested different methods to eliminate cross-reactivities and improve affinity, eventually achieving properties that are comparable to those of the reported monoclonal antibody candidates for the therapy of overdose. Multiple aptamers that we now report share structural motifs with the previously reported receptor for serotonin. Further mutagenesis studies revealed a palindromic, highly adaptable, broadly cross-reactive hydrophobic motif that could be rebuilt through mutagenesis, expansion of linker regions, and selections into receptors with exceptional affinities and varying specificities.

Advancements in Brain Research: The In Vivo/In Vitro Electrochemical Detection of Neurochemicals

Neurochemicals, crucial for nervous system function, influence vital bodily processes and their fluctuations are linked to neurodegenerative diseases and mental health conditions. Monitoring these compounds is pivotal, yet the intricate nature of the central nervous system poses challenges. Researchers have devised methods, notably electrochemical sensing with micro-nanoscale electrodes, offering high-resolution monitoring despite low concentrations and rapid changes. Implantable sensors enable precise detection in brain tissues with minimal damage, while microdialysis-coupled platforms allow in vivo sampling and subsequent in vitro analysis, addressing the selectivity issues seen in other methods. While lacking temporal resolution, techniques like HPLC and CE complement electrochemical sensing's selectivity, particularly for structurally similar neurochemicals. This review covers essential neurochemicals and explores miniaturized electrochemical sensors for brain analysis, emphasizing microdialysis integration. It discusses the pros and cons of these techniques, forecasting electrochemical sensing's future in neuroscience research. Overall, this comprehensive review outlines the evolution, strengths, and potential applications of electrochemical sensing in the study of neurochemicals, offering insights into future advancements in the field.

Interfacing Aptamer-Modified Nanopipettes with Neuronal Media and Ex Vivo Brain Tissue

Aptamer-functionalized biosensors exhibit high selectivity for monitoring neurotransmitters in complex environments. We translated nanoscale aptamer-modified nanopipette sensors to detect endogenous dopamine release in vitro and ex vivo. These sensors employ quartz nanopipettes with nanoscale pores (ca. 10 nm diameter) that are functionalized with aptamers that enable the selective capture of dopamine through target-specific conformational changes. The dynamic behavior of aptamer structures upon dopamine binding leads to the rearrangement of surface charge within the nanopore, resulting in measurable changes in ionic current. To assess sensor performance in real time, we designed a fluidic platform to characterize the temporal dynamics of nanopipette sensors. We then conducted differential biosensing by deploying control sensors modified with nonspecific DNA alongside dopamine-specific sensors in biological milieu. Our results confirm the functionality of aptamer-modified nanopipettes for direct measurements in undiluted complex fluids, specifically in the culture media of human-induced pluripotent stem cell-derived dopaminergic neurons. Moreover, sensor implantation and repeated measurements in acute brain slices was possible, likely owing to the protected sensing area inside nanoscale DNA-filled orifices, minimizing exposure to nonspecific interferents and preventing clogging. Further, differential recordings of endogenous dopamine released through electrical stimulation in the dorsolateral striatum demonstrate the potential of aptamer-modified nanopipettes for ex vivo recordings with unprecedented spatial resolution and reduced tissue damage.

Signal transduction interfaces for field-effect transistor-based biosensors

Biosensors based on field-effect transistors (FETs) are suitable for use in miniaturized and cost-effective healthcare devices. Various semiconductive materials can be applied as FET channels for biosensing, including one- and two-dimensional materials. The signal transduction interface between the biosample and the channel of FETs plays a key role in translating electrochemical reactions into output signals, thereby capturing target ions or biomolecules. In this Review, distinctive signal transduction interfaces for FET biosensors are introduced, categorized as chemically synthesized, physically structured, and biologically induced interfaces. The Review highlights that these signal transduction interfaces are key in controlling biosensing parameters, such as specificity, selectivity, binding constant, limit of detection, signal-to-noise ratio, and biocompatibility.

Rapid isolation of anti-idiotype aptamers for quantification of human monoclonal antibodies against SARS-CoV-2 spike protein

Therapeutic antibodies that block viral entry have already proven to be important, first line drugs for treatments of viral infections. In the case of SARS-CoV-2, combinations of multiple therapeutic antibodies may need to be rapidly identified and formulated in a way that blocks each new, predominant variant of the virus. For efficient introduction of any new antibody combination into patients, it is important to be able to monitor patient-specific pharmacokinetics of individual antibodies, which would include the time course of their specific capacity to block the viral spike proteins. Here, we present three examples of microfluidic-based rapid isolation of companion reagents useful for establishing combination antibody therapies. These reagents are specific three-dimensional imprints of variable regions of individual human monoclonal antibodies against the -spike protein of SARS-CoV-2 virus in the form of oligonucleotide-based ligands (aptamers). We implement these anti-idiotypic aptamers as bioreceptors in graphene-based field-effect transistor sensors to accomplish label free, rapid, and sensitive detection of matching antibodies within minutes. Through this work we have demonstrated the general applicability of anti-idiotype aptamers as capture reagents in quantification of active forms of monoclonal antibodies in complex biological mixtures.

Addressing Signal Drift and Screening for Detection of Biomarkers with Carbon Nanotube Transistors

Electrical biosensors, including transistor-based devices (i.e., BioFETs), have the potential to offer versatile biomarker detection in a simple, low-cost, scalable, and point-of-care manner. Semiconducting carbon nanotubes (CNTs) are among the most explored nanomaterial candidates for BioFETs due to their high electrical sensitivity and compatibility with diverse fabrication approaches. However, when operating in solutions at biologically relevant ionic strengths, CNT-based BioFETs suffer from debilitating levels of signal drift and charge screening, which are often unaccounted for or sidestepped (but not addressed) by testing in diluted solutions. In this work, we present an ultrasensitive CNT-based BioFET called the D4-TFT, an immunoassay with an electrical readout, which overcomes charge screening and drift-related limitations of BioFETs. In high ionic strength solution (1X PBS), the D4-TFT repeatedly and stably detects subfemtomolar biomarker concentrations in a point-of-care form factor by increasing the sensing distance in solution (Debye length) and mitigating signal drift effects. Debye length screening and biofouling effects are overcome using a poly(ethylene glycol)-like polymer brush interface (POEGMA) above the device into which antibodies are printed. Simultaneous testing of a control device having no antibodies printed over the CNT channel confirms successful detection of the target biomarker via an on-current shift caused by antibody sandwich formation. Drift in the target signal is mitigated by a combination of: (1) maximizing sensitivity by appropriate passivation alongside the polymer brush coating; (2) using a stable electrical testing configuration; and (3) enforcing a rigorous testing methodology that relies on infrequent DC sweeps rather than static or AC measurements. These improvements are realized in a relatively simple device using printed CNTs and antibodies for a low-cost, versatile platform for the ongoing pursuit of point-of-care BioFETs.

Selective Hemin Binding by a Non-G-quadruplex Aptamer with Higher Affinity and Better Peroxidase-like Activity

Previous aptamers for porphyrins and metalloporphyrins were all guanine-rich sequences that can fold in G-quadruplex structures. Due to stacking-based binding, these aptamers can hardly tell different porphyrins apart, and they can also bind other planar molecules, hindering their practical applications. In this work, we used the capture selection method to obtain aptamers for hemin and protoporphyrin IX (PPIX). The hemin aptamer (Hem1) features two highly conserved repeating binding loops, and it cannot form a G-quadruplex, which was supported by its Mg2+ -dependent but K+ -independent hemin binding and CD spectroscopy. Isothermal titration calorimetry revealed much higher enthalpy change for the new aptamer, and the best aptamer showed a Kd of 43 nM hemin. Hem1 can also enhance the peroxidase-like activity of hemin. This work demonstrates that aptamers have alternative ways to bind porphyrins allowing selective recognition of different porphyrins.

Detection of miR-155 Using Peptide Nucleic Acid at Physiological-like Conditions by Surface Plasmon Resonance and Bio-Field Effect Transistor

MicroRNAs are small ribonucleotides that act as key gene regulators. Their altered expression is often associated with the onset and progression of several human diseases, including cancer. Given their potential use as biomarkers, there is a need to find detection methods for microRNAs suitable for use in clinical setting. Field-effect-transistor-based biosensors (bioFETs) appear to be valid tools to detect microRNAs, since they may reliably quantitate the specific binding between the immobilized probe and free target in solution through an easily detectable electrical signal. We have investigated the detection of human microRNA 155 (miR-155) using an innovative capturing probe constituted by a synthetic peptide nucleic acid (PNA), which has the advantage to form a duplex even at ionic strengths approaching the physiological conditions. With the aim to develop an optimized BioFET setup, the interaction kinetics between miR-155 and the chosen PNA was preliminarily investigated by using surface plasmon resonance (SPR). By exploiting both these results and our custom-made bioFET system, we were able to attain a low-cost, real-time, label-free and highly specific detection of miR-155 in the nano-molar range.

Recent Advances in Real-Time Label-Free Detection of Small Molecules

The detection and analysis of small molecules, typically defined as molecules under 1000 Da, is of growing interest ranging from the development of small-molecule drugs and inhibitors to the sensing of toxins and biomarkers. However, due to challenges such as their small size and low mass, many biosensing technologies struggle to have the sensitivity and selectivity for the detection of small molecules. Notably, their small size limits the usage of labeled techniques that can change the properties of small-molecule analytes. Furthermore, the capability of real-time detection is highly desired for small-molecule biosensors' application in diagnostics or screening. This review highlights recent advances in label-free real-time biosensing technologies utilizing different types of transducers to meet the growing demand for small-molecule detection.

Toward the Ultimate Limit of Analyte Detection, in Graphene-Based Field-Effect Transistors

The ultimate sensitivity of field-effect-transistor (FET)-based devices for ionic species detection is of great interest, given that such devices are capable of monitoring single-electron-level modulations. It is shown here, from both theoretical and experimental perspectives, that for such ultimate limits to be approached the thermodynamic as well as kinetic characteristics of the (FET surface)-(linker)-(ion-receptor) ensemble must be considered. The sensitivity was probed in terms of optimal packing of the ensemble, through a minimal charge state/capacitance point of view and atomic force microscopy. Through the fine-tuning of the linker and receptor interaction with the sensing surface, a record limit of detection as well as specificity in the femtomolar range, orders of magnitude better than previously obtained and in excellent accord with prediction, was observed.

Aptamer Renaissance for Neurochemical Biosensing

Unraveling the complexities of brain function, which is crucial for advancing human health, remains a grand challenge. This endeavor demands precise monitoring of small molecules such as neurotransmitters, the chemical messengers in the brain. In this Perspective, we explore the potential of aptamers, selective synthetic bioreceptors integrated into electronic affinity platforms to address limitations in neurochemical biosensing. We emphasize the importance of characterizing aptamer thermodynamics and target binding to realize functional biosensors in biological systems. We focus on two label-free affinity platforms spanning the micro- to nanoscale: field-effect transistors and nanopores. Integration of well-characterized structure-switching aptamers overcame nonspecific binding, a challenge that has hindered the translation of biosensors from the lab to the clinic. In a transformative era driven by neuroscience breakthroughs, technological innovations, and multidisciplinary collaborations, an aptamer renaissance holds the potential to bridge technological gaps and reshape the landscape of diagnostics and neuroscience.

Biocompatible Biphasic Iontronics Enable Neuron-Like Ionic Signal Transmission

Biocompatible connections between external artificial devices and living organisms show promise for future neuroprosthetics and therapeutics. The study in Science by Zhao and colleagues introduces a cascade-heterogated biphasic gel (HBG) iontronic device, which facilitates electronic-to-multi-ionic signal transduction for abiotic-biotic interfaces. Inspired by neuron signaling, the HBG device demonstrated its biocompatibility by regulating neural activity in biological tissue, paving the way for wearable and implantable devices, including brain-computer interfaces.

Affinity-Guided Coevolution of Aptamers for Guanine, Xanthine, Hypoxanthine, and Adenine

The simultaneous evolution of multiple aptamers can drastically increase the speed of aptamer discovery. Most previous studies used the same concentration for different targets, leading to the dominance of the libraries by one or a few aptamers and a low success rate. To foster the best aptamers to grow independently in the sequence space, it is important to (1) use low target concentrations close to their dissociation constants and (2) stop at an early round before any sequence starts to dominate. In this study, we demonstrate this affinity-guided selection concept using the capture-SELEX method to isolate aptamers for four important purines: guanine (5 μM), xanthine (50 μM), hypoxanthine (10 μM), and adenine (10 μM). The round 9 library was split, and in round 10, the four targets were individually used to elute the binding sequences. Using thioflavin T fluorescence spectroscopy and isothermal titration calorimetry, we confirmed highly selective aptamers for xanthine, guanine, and adenine. These aptamers have Kd values below 1 μM and around 100-fold selectivity against most competing analytes, and they compare favorably with existing RNA aptamers and riboswitches. A separate selection was performed using hypoxanthine alone, and no selective aptamer was achieved, even with negative selection, explaining the lack of its aptamer in our mixed selection. This affinity-guided multiplex SELEX study offers fundamental insights into aptamer selection and provides high-quality aptamers for three important purines.

Multiplexed immunosensing of cancer biomarkers on a split-float-gate graphene transistor microfluidic biochip

This work reports the development of a novel microfluidic biosensor using a graphene field-effect transistor (GFET) design for the parallel label-free analysis of multiple biomarkers. Overcoming the persistent challenge of constructing μm2-sized FET sensitive interfaces that incorporate multiple receptors, we implement a split-float-gate structure that enables the manipulation of multiplexed biochemical functionalization using microfluidic channels. Immunoaffinity biosensing experiments are conducted using the mixture samples containing three liver cancer biomarkers, carcinoembryonic antigen (CEA), α-fetoprotein (AFP), and parathyroid hormone (PTH). The results demonstrate the capability of our label-free biochip to quantitatively detect multiple target biomarkers simultaneously by observing the kinetics in 10 minutes, with the detection limit levels in the nanomolar range. This microfluidic biosensor provides a valuable analytical tool for rapid multi-target biosensing, which can be potentially utilized for domiciliary tests of cancer screening and prognosis, obviating the need for sophisticated instruments and professional operations in hospitals.

Development of a Smart Wireless Multisensor Platform for an Optogenetic Brain Implant

Implantable cell replacement therapies promise to completely restore the function of neural structures, possibly changing how we currently perceive the onset of neurodegenerative diseases. One of the major clinical hurdles for the routine implementation of stem cell therapies is poor cell retention and survival, demanding the need to better understand these mechanisms while providing precise and scalable approaches to monitor these cell-based therapies in both pre-clinical and clinical scenarios. This poses significant multidisciplinary challenges regarding planning, defining the methodology and requirements, prototyping and different stages of testing. Aiming toward an optogenetic neural stem cell implant controlled by a smart wireless electronic frontend, we show how an iterative development methodology coupled with a modular design philosophy can mitigate some of these challenges. In this study, we present a miniaturized, wireless-controlled, modular multisensor platform with fully interfaced electronics featuring three different modules: an impedance analyzer, a potentiostat and an optical stimulator. We show the application of the platform for electrical impedance spectroscopy-based cell monitoring, optical stimulation to induce dopamine release from optogenetically modified neurons and a potentiostat for cyclic voltammetry and amperometric detection of dopamine release. The multisensor platform is designed to be used as an opto-electric headstage for future in vivo animal experiments.