Electrochemical, photoelectrochemical, and surface plasmon resonance detection of cocaine using supramolecular aptamer complexes and metallic or semiconductor nanoparticles

Delineating a Tailor-Made Fluorescent Probe Designed for the Selective Detection of Tyrosinase

A dicyanoisophorone based fluorescent probe (E)-2-(3-(4-hydroxystyryl)-5,5-dimethylcyclohex-2-en-1-ylidene)malononitrile (DCIP-OH) was developed for the selective sensing of tyrosinase in apple extract and live cells. The probe was obtained by the condensation of 2-(3,5,5-trimethylcyclohex-2-en-1-ylidene)malononitrile with 4-hydroxybenzaldehyde. Upon interaction with tyrosinase, the probe exhibited absorbance switching from 417 nm to 357 nm, accompanied by a slight increase in absorption value and an isosbestic point observed at 373 nm. Additionally, a reduction in emission intensity at 592 nm was observed. Furthermore, we successfully employed the probe for sensing of tyrosinase in apple extract and conducted inhibition studies by using kojic acid. LOD was determined to be ~0.4 nM. Moreover, the biocompatible nature of DCIP-OH enabled its effective localization in epithelial-like melanoma cells, B16F10, where it demonstrated successful fluorescent probing of intracellular tyrosinase.

Application of valence-variable transition-metal-oxide-based nanomaterials in electrochemical analysis: A review

The construction of materials with rapid electron transfer is considered an effective method for enhancing electrochemical activity in electroanalysis. It has been widely demonstrated that valence changes in transition metal ions can promote electron transfer and thus increase electrochemical activity. Recently, valence-variable transition metal oxides (TMOs) have shown popular application in electrochemical analysis by using their abundant valence state changes to accelerate electron transfer during electrochemical detection. In this review, we summarize recent research advances in valence changes of TMOs and their application in electrochemical analysis. This includes the definition and mechanism of valence change, the association of valence changes with electronic structure, and their applications in electrochemical detection, along with the use of density functional theory (DFT) to simulate the process of electron transfer during valence changes. Finally, the challenges and opportunities for developing and applying valence changes in electrochemical analysis are also identified.

Surface immobilization strategies for the development of electrochemical nucleic acid sensors

Following the recent pandemic and with the emergence of cell-free nucleic acids in liquid biopsies as promising biomarkers for a broad range of pathologies, there is an increasing demand for a new generation of nucleic acid tests, with a particular focus on cost-effective, highly sensitive and specific biosensors. Easily miniaturized electrochemical sensors show the greatest promise and most typically rely on the chemical functionalization of conductive materials or electrodes with sequence-specific hybridization probes made of standard oligonucleotides (DNA or RNA) or synthetic analogues (e.g. Peptide Nucleic Acids or PNAs). The robustness of such sensors is mostly influenced by the ability to control the density and orientation of the probe at the surface of the electrode, making the chemistry used for this immobilization a key parameter. This exhaustive review will cover the various strategies to immobilize nucleic acid probes onto different solid electrode materials. Both physical and chemical immobilization techniques will be presented. Their applicability to specific electrode materials and surfaces will also be discussed as well as strategies for passivation of the electrode surface as a way of preventing electrode fouling and reducing nonspecific binding.

Platinum Black/Gold Nanoparticles/Polyaniline Modified Electrochemical Microneedle Sensors for Continuous In Vivo Monitoring of pH Value

Continuous in vivo monitoring (CIVM) of pH value is essential for personalized medicine, as many diseases are closely related to acid-base imbalances. However, conventional pH meters are limited in their ability to perform CIVM due to excessive blood consumption, large device volume, frequent calibration, and inadequate real-time monitoring. There is thus an urgent need for a portable method for CIVM of pH value. To address this need, we propose a minimally invasive, continuous monitoring solution in the form of an implantable pH microneedle sensor (MNS) in this study. The MNS is based on the integration of an acupuncture needle (AN) and a Ag/AgCl reference electrode. We fabricate the sensor by electrochemically depositing platinum black and gold nanoparticles onto the AN and further modifying it with polyaniline to increase its sensitivity to hydrogen ions. The pH value is obtained by calculating the open circuit voltage between the modified AN and the reference electrode. The resulting MNS demonstrates excellent selectivity and a high nernstian response to pH (-57.4 mV per pH) over a broad range (pH = 4.0 to pH = 9.0). Both in vitro and in vivo experiments have verified the performance of the sensor, showcasing its potential for biomedical research and clinical practice. The MNS provides an alternative to conventional pH meters, offering a less invasive and more convenient way to perform CIVM of pH value. Moreover, this electrochemical implantable sensor based on AN and silver wires provides a simple and sensitive method for continuous in vivo detection of other biomarkers.

Semiconductor quantum dots in photoelectrochemical sensors from fabrication to biosensing applications

Semiconductor quantum dots (QDs) are a promising class of nanomaterials for developing new photoelectrodes and photoelectrochemistry systems for energy storage, transfer, and biosensing applications. These materials have unique electronic and photophysical properties and can be used as optical nanoprobes in displays, biosensors, imaging, optoelectronics, energy storage and energy harvesting. Researchers have recently been exploring the use of QDs in photoelectrochemical (PEC) sensors, which involve exciting a QD-interfaced photoactive material with a flashlight source and generating a photoelectrical current as an output signal. The simple surface properties of QDs also make them suitable for addressing issues related to sensitivity, miniaturization, and cost-effectiveness. This technology has the potential to replace current laboratory practices and equipment, such as spectrophotometers, used for testing sample absorption and emission. Semiconductor QD-based PEC sensors offer simple, fast, and easily miniaturized sensors for analyzing a variety of analytes. This review summarizes the various strategies for interfacing QD nanoarchitectures for PEC sensing, as well as their signal amplification. PEC sensing devices, particularly those used for the detection of disease biomarkers, biomolecules (glucose, dopamine), drugs, and various pathogens, have the potential to revolutionize the biomedical field. This review discusses the advantages of semiconductor QD-based PEC biosensors and their fabrication methods, with a focus on disease diagnostics and the detection of various biomolecules. Finally, the review provides prospects and considerations for QD-based photoelectrochemical sensor systems in terms of their sensitivity, speed, and portability for biomedical applications.

Graphene Based Nanohybrid Aptasensors in Environmental Monitoring: Concepts, Design and Future Outlook

In view of ever-increasing environmental pollution, there is an immediate requirement to promote cheap, multiplexed, sensitive and fast biosensing systems to monitor these pollutants or contaminants. Aptamers have shown numerous advantages in being used as molecular recognition elements in various biosensing devices. Graphene and graphene-based materials/nanohybrids combined with several detection methods exhibit great potential owing to their exceptional optical, electronic and physicochemical properties which can be employed extensively to monitor environmental contaminants. For environmental monitoring applications, aptamers have been successfully combined with graphene-based nanohybrids to produce a wide range of innovative methodologies. Aptamers are immobilized at the surface of graphene based nanohybrids via covalent and non-covalent strategies. This review highlights the design, working principle, recent developmental advances and applications of graphene based nanohybrid aptasensors (GNH-Apts) (since January 2014 to September 2021) with a special emphasis on two major signal-transduction methods, i.e., optical and electrochemical for the monitoring of pesticides, heavy metals, bacteria, antibiotics, and organic compounds from different environmental samples (e.g., water, soil and related). Lastly, the challenges confronted by scientists and the possible future outlook have also been addressed. It is expected that high-performance graphene-based nanohybrid aptasensors would find broad applications in the field of environmental monitoring.

Photoelectrochemical IL-6 Immunoassay Manufactured on Multifunctional Catecholate-Modified TiO2 Scaffolds

There is an increasing interest in using photoelectrochemistry for enhancing the signal-to-noise ratio and sensitivity of electrochemical biosensors. Nevertheless, it remains challenging to create photoelectrochemical biosensors founded on stable material systems that are also easily biofunctionalized for sensing applications. Herein, a photoelectrochemical immunosensor is reported, in which the concentration of the target protein directly correlates to a change in the measured photocurrent. The material system for the photoelectrode signal transducer involves using catecholate ligands to modify the properties of TiO₂ nanostructures in a three-pronged approach of morphology tuning, photoabsorption enhancement, and facilitating bioconjugation. The catecholate-modified TiO₂ photoelectrode is combined with a signal-off direct immunoassay to detect interleukin-6 (IL-6), a key biomarker for diagnosing and monitoring various diseases. Catecholate ligands are added during hydrothermal synthesis of TiO₂ to enable the growth of three-dimensional nanostructures to form highly porous photoelectrodes that provide a three-dimensional scaffold for immobilizing capture antibodies. Surface modification by catecholate ligands greatly enhances photocurrent generation of the TiO₂ photoelectrodes by improving photoabsorption in the visible range. Additionally, catecholate molecules facilitate bioconjugation and probe immobilization by forming a Schiff-base between their -COH group and the -NH₂ group of the capture antibodies. The highest photocurrent achieved herein is 8.89 μA cm^-2, which represents an enhancement by a factor of 87 from unmodified TiO₂. The fabricated immunosensor shows a limit-of-detection of 3.6 pg mL^-1 and a log-linear dynamic range of 2-2000 pg mL^-1 for IL-6 in human blood plasma.

Light in Electrochemistry

Electrochemistry represents an important analytical technique used to acquire and assess chemical information in detail, which can aid fundamental investigations in various fields, such as biological studies. For example, electrochemistry can be used as simple and cost-effective means for bio-marker tracing in applications, such as health monitoring and food security screening. In combination with light, powerful spatially-resolved applications in both the investigation and manipulation of biochemical reactions begin to unfold. In this article, we focus primarily on light-addressable electrochemistry based on semiconductor materials and light-readable electrochemistry enabled by electrochemiluminescence (ECL). In addition, the emergence of multiplexed and imaging applications will also be introduced.

Paper-based microfluidic aptasensors

For in-situ disease markers detection, point-of-care (POC) diagnosis has great advantages in speed and cost compared with traditional techniques. The rapid diagnosis, prognosis, and surveillance of diseases can significantly reduce disease-related mortality and trauma. Therefore, increasing attention has been paid to the POC diagnosis devices due to their excellent diagnosis speed and portability. Over the past ten years, paper-based microfluidic aptasensors have emerged as a class of critical POC diagnosis devices and various aptasensors have been proposed to detect various disease markers. However, most aptasensors need further improvement before they can actually enter the market and be widely used. There is thus an urgent need to sort out the key points of preparing the aptasensors and the direction that needs to be invested in. This review summarizes the representative articles in the development of paper-based microfluidic aptasensors. These works can be divided into paper-based optical aptasensors and paper-based electrochemical aptasensors according to their output signals. Significant focus is applied to these works according to the following three parts: (1) The ingenious design of device structure; (2) Application and synthesis of nanomaterial; (3) The detection principle of the proposed aptasensor. This is a detailed and comprehensive review of paper-based microfluidic aptasensors. The accomplishments and shortcomings of the current aptasensors are outlined, the development direction and the future prospective are given. It is hoped that the research in this review can provide a reference for further development of more advanced, more effective paper-based microfluidic aptasensors for POC disease markers diagnosis.

Aptasensors for Point-of-Care Detection of Small Molecules

Aptamers, a group of nucleic acids which can specifically bind to a target molecule, have drawn extensive interest over the past few decades. For analytics, aptamers represent a viable alternative to gold-standard antibodies due to their oligonucleic nature combined with advantageous properties, including higher stability in harsh environments and longer shelf-life. Indeed, over the last decade, aptamers have been used in numerous bioanalytical assays and in various point-of-care testing (POCT) platforms. The latter allows for rapid on-site testing and can be performed outside a laboratory by unskilled labor. Aptamer technology for POCT is not limited just to medical diagnostics; it can be used for a range of applications, including environmental monitoring and quality control. In this review, we critically examine the use of aptamers in POCT with an emphasis on their advantages and limitations. We also examine the recent success of aptasensor technology and how these findings pave the way for the analysis of small molecules in POCT and other health-related applications. Finally, the current major limitations of aptamers are discussed, and possible approaches for overcoming these challenges are presented.

Coordination-driven assembly of a 3d-4f heterometallic organic framework with 1D Cu4I4 and Eu-based chains: syntheses, structures and various properties

A three-dimensional porous 3d-4f heterometallic organic framework, namely, {[Eu3(Cu4I4)3(INA)9(DMF)4]·3DMF}n (YNU-2), was successfully prepared under solvothermal conditions. There are two different one-dimensional metal chains in the structure, namely, Cu4I4 and EuIII-based chains, resulting in an excellent stability of the prepared sample. A N2 sorption isotherm at 77 K revealed that the activated sample exhibits a Brunauer-Emmett-Teller surface area of 371 m2 g-1, while, YNU-2 can adsorb obviously higher CO2 amounts than CH4 at 273 K and 298 K under 1 atm because of the stronger interaction force between CO2 and the porous skeleton. Furthermore, YNU-2 is highly efficient heterogeneous catalyst for chemical fixation of the CO2 and epoxides into cyclic carbonates with a preferable recyclability. Taking into account its excellent stability, the prepared sample can be used to construct an electrochemical adapter sensor for detecting cocaine with a detection limit of 0.27 pg mL-1 in the wide range of 0.001-0.5 ng mL-1.

Ultrasensitive covalently-linked Aptasensor for cocaine detection based on electrolytes-induced repulsion/attraction of colloids

A quick and easy colorimetric sensor based on gold nanoparticles (GNPs) and aptamers for the detection of cocaine was developed. The sensor was named as 'GAPTA' and showed extremely interesting results regarding cocaine detection with a sensitivity to doses of 0.2 nM. The experimental approach consisted of creating a conjugate between GNPs (10 nm size) and aptamers as a sensing base with the addition of an electrolyte (NaCl) that plays the role of aggregation inducer. In the absence of the aptamer, the electrolyte was able to induce aggregation of the GNPs turning the color of the solution from red to blue while the presence of the aptamer is able to hinder the charges attraction and protects the GNPs from aggregating. The optimization of the aptamer and electrolyte concentration was determined to be 118 nM and 55 mM, respectively, and the resultant GAPTA sensor had a detection limit of 0.97 nM. Furthermore, the selectivity of the platform was tested in the presence of different interferents and showed a specific response towards cocaine while interference ranged between 20 and 40%. The applicability of the GAPTA biosensor was tested on synthetic saliva and demonstrated a sensitivity range between 0.2 and 25 nM. These results suggest the potential of the current colorimetric sensor in abuse drugs screening and creates a stable base for new routine platforms for biomedical and toxicology applications. Graphical abstract.

An antibody-aptamer sandwich cathodic photoelectrochemical biosensor for the detection of progesterone

The progesterone (P4) level in body fluids can act as an indicator for early pregnancy diagnosis and offers insight into mammalian somatic function. In this work, we designed an antibody-aptamer based sandwich assay as a cathodic photoelectrochemical (PEC) biosensor for P4 detection. The composites of carbon dots and graphene oxide (CDs-GO) with favorable cathodic photocurrent response were used as photoactive materials on which the antibody (Ab) of P4 was immobilized. Meanwhile, high affinity truncated P4 aptamer was immobilized on Au-CuO-Cu₂O to act as a bioconjugate. When P4 was present, the aptamer-Au-CuO-Cu₂O bioconjugate could amplify the cathodic photocurrent of CDs-GO modified electrode through Ab-P4-aptamer interactions. Under optimum conditions, the cathodic photocurrent of the constructed PEC biosensor was found to increase linearly with P4 in a wide concentration range from 0.5 nM to 180 nM, with a low detection limit (3S/N) of 0.17 nΜ. The proposed cathodic PEC sensing platform demonstrated high selectivity, satisfying reproducibility, good stability. The sensor was successfully applied in the determination of P4 in human serum samples.

Highly sensitive host-guest mode homogenous electrochemical thrombin signal amplification aptasensor based on tetraferrocene label

The development of sensitive and convenient detection methods to monitor thrombin without the use of enzymes or complex nanomaterials is highly desirable for the diagnosis of cardiovascular diseases. In this article, tetraferrocene was first synthesized and then a sensitive and homogeneous electrochemical aptasensor was developed for thrombin detection based on host-guest recognition between tetraferrocene and β-cyclodextrin (β-CD). In the absence of thrombin, the double stem-loop of thrombin aptamer (TBA) prevented tetraferrocenes labeled at both ends from entering the cavity of β-CD deposited on gold electrode surface. After binding with thrombin, the stem-loop structure of TBA opened and transformed into special G-quarter structure, forcing tetraferrocene into the cavity of β-CD. As a result, thrombin allowed eight ferrocene molecules to reach the gold electrode surface, greatly amplifying the response signal. The obtained aptasensors showed dynamic detection range from 4 pM to 12.5 nM with detection limit around 1.2 pM. Overall, the results indicate that the proposed aptasensors are promising for future rapid clinical detection of thrombin and development of signal amplification strategies for detection of various proteins.

3D hollow-out TiO2 nanowire cluster/GOx as an ultrasensitive photoelectrochemical glucose biosensor

Ultra-high sensitivity is difficult to achieve using conventional enzymatic glucose biosensors due to the lack of exposed active sites and steric-hinderance effects. Thus, in the present study, we report a photoelectrochemical (PEC) enzymatic glucose biosensor based on 3-dimensional (3D) hollow-out titanium dioxide (TiO₂) nanowire cluster (NWc)/glucose oxidase (GOx), providing more number of exposed active sites, thus constructing a sensor with a higher affinity toward glucose reaction. Excellent performance with an ultra-high sensitivity of 58.9 μA mM^-1 cm^-2 and 0-2 mM linear range with a determination limit of 8.7 μM was obtained for the detection of glucose. This study might provide a new approach to expose active sites efficiently for remarkable photoelectrochemical performances.

A pro-gastrin-releasing peptide imprinted photoelectrochemical sensor based on the in situ growth of gold nanoparticles on a MoS2 nanosheet surface

Molecularly imprinted PEC platform for Pro-GRP sensing was prepared using lamellar MoS₂ nanosheets assembled with gold nanoparticles as photoactive elements. The molecularly imprinted PEC sensor shows excellent sensing performances towards Pro-GRP.

Nanomaterial-based aptamer sensors for arsenic detection

Arsenic (As) is a highly toxic contaminant in the environment and a serious carcinogen for the human being. The toxicity of arsenic significantly threatens environmental and human health. The effective removing technology for arsenic remains challenging, and one of the reasons is due to the lack of powerful detection method in the complex environmental matrix. There is thus an urgent need to develop novel analytical methods for arsenic, preferably with the potential for the field-testing. To combat arsenic pollution and maintain a healthy environment and eco-system, many analytical methods have been developed for arsenic detection in various samples. Among these strategies, biosensors hold great promise for rapid detection of arsenic, in particular, nanomaterials-based aptamer sensors have attracted significant attention due to their simplicity, high sensitivity and rapidness. In this paper, we reviewed the recent development and applications of aptamer sensors (aptasensors) based-on nanomaterial for arsenic detection, in particular with emphasis on the works using optical and electrochemical technologies. We also discussed the recent novel technology in aptasensors development for arsenic detection, including nucleic acid amplification for signal enhancement and device integration for the portability of arsenic sensors. We are hoping this review could inspire further researches in developing novel nanotechnologies based aptasensors for possible on-site detection of arsenic.

A Bottom-Up Approach for Developing Aptasensors for Abused Drugs: Biosensors in Forensics

Aptamer-based point-of-care (POC) diagnostics platforms may be of substantial benefit in forensic analysis as they provide rapid, sensitive, user-friendly, and selective analysis tools for detection. Aptasensors have not yet been adapted commercially. However, the significance of the applications of aptasensors in the literature exceeded their potential. Herein, in this review, a bottom-up approach is followed to describe the aptasensor development and application procedure, starting from the synthesis of the corresponding aptamer sequence for the selected analyte to creating a smart surface for the sensitive detection of the molecule of interest. Optical and electrochemical biosensing platforms, which are designed with aptamers as recognition molecules, detecting abused drugs are critically reviewed, and existing and possible applications of different designs are discussed. Several potential disciplines in which aptamer-based biosensing technology can be of greatest value, including forensic drug analysis and biological evidence, are then highlighted to encourage researchers to focus on developing aptasensors in these specific areas.

Affinity-Based Detection of Biomolecules Using Photo-Electrochemical Readout

Detection and quantification of biologically-relevant analytes using handheld platforms are important for point-of-care diagnostics, real-time health monitoring, and treatment monitoring. Among the various signal transduction methods used in portable biosensors, photoelectrochemcial (PEC) readout has emerged as a promising approach due to its low limit-of-detection and high sensitivity. For this readout method to be applicable to analyzing native samples, performance requirements beyond sensitivity such as specificity, stability, and ease of operation are critical. These performance requirements are governed by the properties of the photoactive materials and signal transduction mechanisms that are used in PEC biosensing. In this review, we categorize PEC biosensors into five areas based on their signal transduction strategy: (a) introduction of photoactive species, (b) generation of electron/hole donors, (c) use of steric hinderance, (d) in situ induction of light, and (e) resonance energy transfer. We discuss the combination of strengths and weaknesses that these signal transduction systems and their material building blocks offer by reviewing the recent progress in this area. Developing the appropriate PEC biosensor starts with defining the application case followed by choosing the materials and signal transduction strategies that meet the application-based specifications.

Amplified oxygen reduction signal at a Pt-Sn-modified TiO2 nanocomposite on an electrochemical aptasensor

In this work, a metallic composite with strong electrocatalytic property was designed by uniformly decorating Pt and Sn nanoparticles on the surface of TiO₂ nanorods (Pt-Sn@TiO₂). A detection scheme was then developed based on a dual signal amplification strategy involving the Pt-Sn@TiO₂ composite and exonuclease assisted target recycling. The Pt-Sn@TiO₂ composite exhibited an enhanced oxygen reduction current owing to the synergistic effect between Pt and Sn, as well as high exposure of Pt (111) crystal face. Initially, a Pt-Sn@TiO₂ modified glassy carbon electrode produced an amplified electrochemical signal for the reduction of dissolved oxygen in the analyte solution. Next, a DNA with a complementary sequence to a streptomycin aptamer (cDNA) was immobilised on the Pt-Sn@TiO₂ modified electrode, followed by the streptomycin aptamer that hybridised with cDNA. The corresponding oxygen reduction current was diminished by 51% attributable to the hindrance from the biomolecules. After a mixture of streptomycin and RecJ_f exonuclease was introduced, both the streptomycin-aptamer complex and the cDNA were cleaved from the electrode, making the Pt-Sn and Pt (111) surface available for oxygen reduction. RecJ_f would also release streptomycin from the streptomycin-aptamer complex, allowing it to complex again with aptamers on the electrode. This has then promoted a cyclic amplification of the oxygen reduction current by 85%, which is quantitatively related to streptomycin. Under optimal conditions, the aptasensor exhibited a linear range of 0.05-1500 nM and a limit of detection of 0.02±0.0045 nM streptomycin. The sensor was then used in the real-life sample detection of streptomycin to demonstrate its potential applications to bioanalysis.

Electrochemical aptasensor based on conductive supramolecular polymer hydrogels for thrombin detection with high selectivity

Herein, we synthesized a kind of conductive supramolecular polymer hydrogel (CSPH) based on polyaniline (PANI) which can not only improve the conductivity but also promote antifouling performance of the aptasensor for the specific recruitment of thrombin (TB) from complex samples. With the electrochemical copolymerization of aniline (AN) and 3-aminophenylboronic acid (ABA) on glassy carbon electrode (GCE), the electrode was then inserted into the polyvinyl alcohol (PVA) solution to obtain robust CSPH through boric acid groups incorporated onto PANI to cause gelation of PVA solution, owing to the hydrophilicity of CSPH and nearly electrical neutrality, the modified electrode is antifouling without integration of other antifouling materials. A sandwich-type electrochemical aptasensor was constructed on the CSPH based electrode interfaces. Thrombin aptamer 1 (TBA1) were modified on the CSPH through amide bond, and thrombin aptamer 2 modified magnetic nanoparticles (MNP-TBA2) are used as signal amplification probes, the aptasensor has good sensitivity with a linear range from 1 pmol/L to 10 nmol/L and has a detection limit down to 0.64 pmol/L. The strategy of utilizing eletropolymerization of CSPH films to undergo highly selective thrombin recognition is, of course, readily extended to a broad range of targets in the real samples, and the recovery was ranging from 95.2% to 106.3% and RSDs varying from 2.3% to 4.5%.

Optical Biosensors for Label-Free Detection of Small Molecules

Label-free optical biosensors are an intriguing option for the analyses of many analytes, as they offer several advantages such as high sensitivity, direct and real-time measurement in addition to multiplexing capabilities. However, development of label-free optical biosensors for small molecules can be challenging as most of them are not naturally chromogenic or fluorescent, and in some cases, the sensor response is related to the size of the analyte. To overcome some of the limitations associated with the analysis of biologically, pharmacologically, or environmentally relevant compounds of low molecular weight, recent advances in the field have improved the detection of these analytes using outstanding methodology, instrumentation, recognition elements, or immobilization strategies. In this review, we aim to introduce some of the latest developments in the field of label-free optical biosensors with the focus on applications with novel innovations to overcome the challenges related to small molecule detection. Optical label-free methods with different transduction schemes, including evanescent wave and optical fiber sensors, surface plasmon resonance, surface-enhanced Raman spectroscopy, and interferometry, using various biorecognition elements, such as antibodies, aptamers, enzymes, and bioinspired molecularly imprinted polymers, are reviewed.

A new electrochemical aptasensor for sensitive assay of a protein based on the dual-signaling electrochemical ratiometric method and DNA walker strategy

Herein, a new electrochemical aptamer-based biosensor for highly sensitive assay of thrombin has been developed based on the dual-signaling electrochemical ratiometric method and the DNA walker strategy, and shows a low detection limit of about 56 fM.

Design of a simple and novel photoelectrochemical aptasensor for detection of 3,3',4,4'-tetrachlorobiphenyl

In view of the urgent need of determining polychlorinated biphenyls in the environment, we developed a highly sensitive and selective photoelectrochemical (PEC) aptasensor for determination of 3,3',4,4'-tetrachlorobiphenyl (PCB77) by immobilizing aptamer on N-doped TiO₂ nanotubes (N-doped TiO₂ NTs). To improve analytical performance of the PEC sensor, the complementary DNA functionalized CdS quantum dots (DNA-CdS QDs) were introduced onto N-doped TiO₂ NTs by hybridization. In addition of PCB77, owing to high affinity of aptamer to PCB77, PCB77-aptamer complexes were formed by being bound of PCB77 whilst DNA-CdS QDs were released from the sensing surface. The complexes with poor conductivity hindered the interfacial electron transfer, leading to the photocurrent decrease. The more important is the release of DNA-CdS QDs enhanced the photocurrent decrease, playing the role of signal amplification. The photocurrent change was utilized to detect PCB77 quantitatively. The PEC aptasensor exhibited excellent analytical performance for detection of PCB77 with wide linear range of 0.1-100 ng/L and a low detection limit of 0.1 ng/L. It manifested outstanding selectivity for PCB77 in control experiments by employing six interferents which had similar structure or coexisted with PCB77. Besides, the PEC aptasensor was used to detect the content of PBC77 in the environment.

A sensitive Potentiometric resolved ratiometric Photoelectrochemical aptasensor for Escherichia coli detection fabricated with non-metallic nanomaterials

In this work, a sensitive potentiometric resolved ratiometric photoelectrochemical aptasensor for Escherichia coli (E. coli) detection was successfully fabricated with non-metallic nanomaterials. To avoid the use of precious metals or heavy metals, three-dimensional graphene hydrogel-loaded carbon quantum dots (C-dots/3DGH) and graphene-like carbon nitride (g-C₃N₄) with excellent PEC activity and matched potential were prepared. These two materials were modified onto two adjacent areas on the ITO electrode. By applying different bias voltage, the cathodic current generated by C-dots/3DGH and the anodic current generated by g-C₃N₄ can be clearly distinguished and would not interfere with one another. Then E. coli aptamer was modified onto the surface of C-dots/3DGH. In the presence of targets, the binding of E. coli with aptamer lead to the steric hindrance greatly increased and the cathodic current decreased significantly. Meanwhile, the anodic current generated by g-C₃N₄ was not influenced and it can serve as a stable reference to evaluate the environmental factors. Therefore, the concentration of E. coli can be quantified by the ratio of cathodic current to anodic current, which can effectively eliminate these analyte-independent factors and provide a more precise analysis. In addition, this ratiometric PEC biosensor also showed a good sensitivity and a wide linear range (2.9 cfu/mL to 2.9 × 10⁶ cfu/mL).

A novel colorimetric aptasensor for ultrasensitive detection of cocaine based on the formation of three-way junction pockets on the surfaces of gold nanoparticles

Herein, a novel colorimetric aptasensor was introduced for detection of cocaine based on the formation of three-way junction pockets on the surfaces of gold nanoparticles (AuNPs) and the catalytic activity of the surfaces of AuNPs. Simplicity and detection of cocaine in a short time (only 35 min) are some of the unique features of the proposed sensing strategy. In the presence of cocaine, triple-fragment aptamer (TFA) forms on the surfaces of AuNPs, leading to a significant decrease of the catalytic activity of AuNPs and the color of samples remains yellow. In the absence of target, TFA does not form on the surfaces of AuNPs and 4-Nitrophenol, as a colorimetric agent, has more access to the surfaces of AuNPs, resulting in the reduction of 4-Nitrophenol and the color of sample changes from yellow to colorless. The sensing strategy showed good specificity, a limit of detection (LOD) of 440 pM and a dynamic range over 2-100 nM. The sensing method was also successfully applied to detect cocaine in spiked human serum samples with recovery of 94.71-98.63%.

Cu2+ -Modified Metal-Organic Framework Nanoparticles: A Peroxidase-Mimicking Nanoenzyme

The synthesis and characterization of UiO-type metal-organic framework nanoparticles (NMOFs) composed of Zr⁴⁺ ions bridged by 2,2'-bipyridine-5,5'-dicarboxylic acid ligands and the postmodification of the NMOFs with Cu²⁺ ions are described. The resulting Cu²⁺ -modified NMOFs, Cu²⁺ -NMOFs, exhibit peroxidase-like catalytic activities reflected by the catalyzed oxidation of Amplex-Red to the fluorescent Resorufin by H₂ O₂ , the catalyzed oxidation of dopamine to aminochrome by H₂ O₂ , and the catalyzed generation of chemiluminescence in the presence of luminol/H₂ O₂ . Also, the Cu²⁺ -NMOFs mimic NADH peroxidase functions and catalyze the oxidation of dihydronicotinamide adenine dinucleotide, NADH, to nicotinamide adenine dinucleotide, NAD⁺ , in the presence of H₂ O₂ . The Cu²⁺ -NMOFs-catalyzed generation of chemiluminescence in the presence of luminol/H₂ O₂ is used to develop a glucose sensor by monitoring the H₂ O₂ formed by the aerobic oxidation of glucose to gluconic acid in the presence of glucose oxidase. Furthermore, loading the Cu²⁺ -NMOFs with fluorescein and activating the catalyzed generation of chemiluminescence in the presence of luminol/H₂ O₂ yield an efficient chemiluminescence resonance energy transfer (CRET) process to the fluorescein reflected by the activation of the fluorescence of the dye (λ = 520 nm, CRET efficiency 35%).

Single Nanochannel-Aptamer-Based Biosensor for Ultrasensitive and Selective Cocaine Detection

Ultrasensitive and selective detection of molecules at nano or sub-nanomolar level is very important for many areas such as early diagnosis and drug testing. Herein, we report a high-sensitive cocaine sensor based on a single nanochannel coupled with DNA aptamers. The single nanochannel-aptamer-based biosensor can recognize cocaine molecules with an excellent sensitivity and good selectivity. A linear relationship between target cocaine concentration and output ionic current is obtained in a wide concentration range of cocaine from 1 nM to 10 μM. The cocaine sensor also shows a detection limit down to 1 nM. This study provides a new avenue to develop new nanochannel-aptamer-based biosensors for rapid and ultratrace detection of a variety of illicit drugs.

Gold Nanoparticles Decorated Hematite Photoelectrode for Sensitive and Selective Photoelectrochemical Aptasensing of Lysozyme

Photoelectrochemical aptasensor (PECAS) is a new and promising detection platform with both high sensitivity and good selectivity. Exploration of new photoelectrode materials and establishment of effective charge transfer channel between photoelectrode and aptamer are the main challenges in this field. In this work, an efficient PECAS based on Au nanoparticles (NPs) decorated Fe₂O₃ nanorod photoelectrode is rationally designed, fabricated, and exhibited excellent sensitivity and selectivity for detection of lysozyme (Lys) with an ultralow detection limit of 3 pM and wide detection range from 10 pM to 100 nM. The Au NPs not only act as anchor to establish an efficient charge transfer channel between the photoelectrode and the aptamer, but also help to enhance the PEC performance through adjusting the carrier density of Fe₂O₃. The rationally designed photoelectrode opens up a distinctive avenue for promoting the PECAS to be a versatile analysis method.

An aptamer-based paper microfluidic device for the colorimetric determination of cocaine

A method utilizing paper microfluidics coupled with gold nanoparticles and two anticocaine aptamers has been developed to detect seized cocaine samples. The ready-to-use format involves the use of a paper strip that produces a color change resulting from the salt-induced aggregation of gold nanoparticles producing a visible color change indicating the presence of the drug. This format is specific for the detection of cocaine. The visual LOD for the method was 2.5 μg and the camera based LOD was 2.36 μg. The operation of the device is easy and rapid, and does not require extensive training or instrumentation. All of the materials utilized in the device are safe and environmental friendly. This device should prove a useful tool for the screening of forensic samples.

Two-Dimensional Zirconium-Based Metal-Organic Framework Nanosheet Composites Embedded with Au Nanoclusters: A Highly Sensitive Electrochemical Aptasensor toward Detecting Cocaine

Two-dimensional (2D) zirconium-based metal-organic framework nanosheets embedded with Au nanoclusters (denoted as 2D AuNCs@521-MOF) were prepared via a one-pot method under mild conditions. The optimized 2D AuNCs@521-MOF nanosheets not only possessed high specific surface area, physicochemical stability, and good electrochemical activity but also exhibited strong bioaffinity toward biomolecule-bearing phosphate groups. Consequently, a large amount of cocaine aptamer strands can be immobilized onto the substrate modified by 2D AuNCs@521-MOF nanosheet, further leading to the formation of a constructed biosensitive platform, which can be used to successfully detect cocaine through the specific binding interactions between cocaine and aptamer strands. The results demonstrated that the 2D AuNCs@521-MOF-based aptasensor had high sensitivity for detecting cocaine within the broad concentration range of 0.001-1.0 ng·mL^-1 and the low limit of detection of 1.29 pM (0.44 pg·mL^-1) and 2.22 pM (0.75 pg·mL^-1) as determined by electrochemical impedance spectroscopy and differential pulse voltammetry, respectively. As expected, with the advantages of high selectivity, repeatability, stability, and simple operation, this new strategy is believed to exhibit great potential for simple and convenient detection of cocaine.

Aptamer-Modified Semiconductor Quantum Dots for Biosensing Applications

Semiconductor quantum dots have attracted extensive interest in the biosensing area because of their properties, such as narrow and symmetric emission with tunable colors, high quantum yield, high stability and controllable morphology. The introduction of various reactive functional groups on the surface of semiconductor quantum dots allows one to conjugate a spectrum of ligands, antibodies, peptides, or nucleic acids for broader and smarter applications. Among these ligands, aptamers exhibit many advantages including small size, high chemical stability, simple synthesis with high batch-to-batch consistency and convenient modification. More importantly, it is easy to introduce nucleic acid amplification strategies and/or nanomaterials to improve the sensitivity of aptamer-based sensing systems. Therefore, the combination of semiconductor quantum dots and aptamers brings more opportunities in bioanalysis. Here we summarize recent advances on aptamer-functionalized semiconductor quantum dots in biosensing applications. Firstly, we discuss the properties and structure of semiconductor quantum dots and aptamers. Then, the applications of biosensors based on aptamer-modified semiconductor quantum dots by different signal transducing mechanisms, including optical, electrochemical and electrogenerated chemiluminescence approaches, is discussed. Finally, our perspectives on the challenges and opportunities in this promising field are provided.

Thrombin aptasensor enabled by Pt nanoparticles-functionalized Co-based metal organic frameworks assisted electrochemical signal amplification

In this work, a Pt nanoparticles-functionalized Co-based metal organic frameworks (PtNPs@Co(II)MOFs@PtNPs) was synthesized and applied in electrochemical aptasensor for thrombin (TB) detection. First, the Co(II)MOFs@PtNPs were prepared via the mixed solvothermal method, which consists of inner Pt nanoparticles (PtNPs) encapsulated by aminofunctionalized Co(II)MOFs materials. Following that, additional PtNPs were adsorbed on the surface of Co(II)MOFs@PtNPs, resulting in the formation of PtNPs@Co(II)MOFs@PtNPs nanocomposite. The PtNPs@Co(II)MOFs@PtNPs nanocomposites with a large surface area were implimented as nanocarriers to immobilize a mass of TBA II for the formation of the TBA II bioconjugates that could be captured onto the electrode surface by sandwich-type format. Moreover, the PtNPs@Co(II)MOFs@PtNPs nanocomposites could directly use as redox tags for charge-generating and electron-transporting with the electron transfer from Co(II) to Co(III). Furthermore, in the presence of H₂O₂, the PtNPs@Co(II)MOF@PtNPs could effectively catalyze H₂O₂ oxidation with improvement electron transfer of redox probe, resulting in electrochemical signal amplification. Based on the above superior advantages, TB was determined in the concentration range from 0.1pM to 50nM with a detection limit of 0.33fM. Furthermore, the excellent sensitivity and selectivity can be easily established for quantitative analysis of other analytes.

Quantum-dots-based photoelectrochemical bioanalysis highlighted with recent examples

Photoelectrochemical (PEC) bioanalysis is a newly developed methodology that provides an exquisite route for innovative biomolecular detection. Quantum dots (QDs) are semiconductor nanocrystals with unique photophysical properties that have attracted tremendous attentions among the analytical community. QDs-based PEC bioanalysis comprises an important research hotspot in the field of PEC bioanalysis due to its combined advantages and potentials. Currently, it has ignited increasing interests as demonstrated by increased research papers. This review aims to cover the most recent advances in this field. With the discussion of recent examples of QDs-PEC bioanalysis from the literatures, special emphasis will be placed on work reporting on fundamental advances in the signaling strategies of QDs-based PEC bioanalysis from 2013 to now. Future prospects in this field are also discussed.