Ultrasensitive Label Free Electrical Detection of Insulin in Neat Blood Serum

An antifouling electrochemical aptasensor based on a polydopamine-polyzwitterion copolymer for tetracycline analysis

Matrix interference resulting from the nonspecific adsorption of non-target components, particularly proteins (fouling), onto sensor surfaces poses a persistent challenge in electrochemical detection of food hazards. The development of antifouling sensor surfaces presents a viable approach to mitigate nonspecific adsorption. In this study, a novel antifouling electrochemical aptasensor, utilizing a zwitterionic polymer, was developed for the sensitive, accurate, and selective detection of tetracycline (TC) in milk. This sensor employs a poly (dopamine)-poly (sulfobetaine methacrylate) (PDA-PSBMA) antifouling copolymer, which is synthesized through an in-situ initiated copolymerization of dopamine on the sensor's surface. Subsequently, the thiol-containing aptamers were immobilized onto the PDA-PSBMA coating through a Michael addition reaction with the poly(dopamine). The resulting antifouling electrochemical aptasensor exhibited robust antifouling performance in various single protein solutions and diluted milk samples, coupled with sensitive and selective recognition of TC. The sensor demonstrated a broad linear response range of 0.1-1000.0 ng mL-1 and a low limit of detection (LOD) of 68.0 pg mL-1. The antifouling electrochemical aptasensor proved effective in assaying TC in diluted milk, with recoveries ranging from 100.0 % to 104.4 %, eliminating the need for additional pretreatments due to its exceptional resistance to nonspecific adhesion.

Ratiometric antifouling electrochemical biosensors based on designed Y-shaped peptide and MXene loaded with Au@ZIF-67 and methylene blue

Based on the designed inverted Y-shaped peptide and MXene nanocomposite (MXene-Au@ZIF-67), a ratiometric anti-pollution electrochemical biosensor was designed and applied to the detection of biomarkers in serum. Au@ZIF-67 inserted into the interior of MXene can not only prevent the accumulation of MXene but also provide a large amounts of binding sites for capturing biomolecules. A designed multifunctional Y-shaped peptide containing anchoring, antifouling, and recognition sequences was anchored onto MXene-Au@ZIF-67 through Au-S bonds. Electrochemical signal molecules, ferrocenecarboxylic acid (Fc) and methylene blue (MB), were modified to another end of multifunctional peptide and interior of MXene-Au@ZIF-67, respectively, to produce a ratiometric electrochemical signal. We selected prostate specific antigen (PSA) as the model compound. PSA specifically recognizes and cleaves the recognition segment in the Y-shaped peptide, and the signal of Fc is reduced, while the signal of MB remains unchanged. The ratiometric strategy endows the present biosensor high accuracy and sensitivity with a detection limit of 0.85 pg/mL. In addition, the sensing surface has good antifouling ability due to the antifouling sequence of the two branching parts of the Y-shaped peptide. More importantly, by replacing the recognition segment of peptides also other targets are accessible, indicating the potential application of the universal detection strategy to the detection of various biomarkers in clinical diagnosis.

Electrochemical Biosensors for Whole Blood Analysis: Recent Progress, Challenges, and Future Perspectives

Whole blood, as one of the most significant biological fluids, provides critical information for health management and disease monitoring. Over the past 10 years, advances in nanotechnology, microfluidics, and biomarker research have spurred the development of powerful miniaturized diagnostic systems for whole blood testing toward the goal of disease monitoring and treatment. Among the techniques employed for whole-blood diagnostics, electrochemical biosensors, as known to be rapid, sensitive, capable of miniaturization, reagentless and washing free, become a class of emerging technology to achieve the target detection specifically and directly in complex media, e.g., whole blood or even in the living body. Here we are aiming to provide a comprehensive review to summarize advances over the past decade in the development of electrochemical sensors for whole blood analysis. Further, we address the remaining challenges and opportunities to integrate electrochemical sensing platforms.

Biomolecular sensors for advanced physiological monitoring

Body-based biomolecular sensing systems, including wearable, implantable and consumable sensors allow comprehensive health-related monitoring. Glucose sensors have long dominated wearable bioanalysis applications owing to their robust continuous detection of glucose, which has not yet been achieved for other biomarkers. However, access to diverse biological fluids and the development of reagentless sensing approaches may enable the design of body-based sensing systems for various analytes. Importantly, enhancing the selectivity and sensitivity of biomolecular sensors is essential for biomarker detection in complex physiological conditions. In this Review, we discuss approaches for the signal amplification of biomolecular sensors, including techniques to overcome Debye and mass transport limitations, and selectivity improvement, such as the integration of artificial affinity recognition elements. We highlight reagentless sensing approaches that can enable sequential real-time measurements, for example, the implementation of thin-film transistors in wearable devices. In addition to sensor construction, careful consideration of physical, psychological and security concerns related to body-based sensor integration is required to ensure that the transition from the laboratory to the human body is as seamless as possible.

Rational design of smart nano-platforms based on antifouling-nanomaterials toward multifunctional bioanalysis

The ability to design nanoprobe devices with the capability of quantitative/qualitative operation in complex media will probably underpin the main upcoming progress in healthcare research and development. However, the biomolecules abundances in real samples can considerably alter the interface performance, where unwanted adsorption/adhesion can block signal response and significantly decrease the specificity of the assay. Herein, this review firstly offers a brief outline of several significances of fabricating high-sensitivity and low-background interfaces to adjust various targets' behaviors induced via bioactive molecules on the surface. Besides, some important strategies to resist non-specific protein adsorption and cell adhesion, followed by imperative categories of antifouling reagents utilized in the construction of high-performance solid sensory interfaces, are discussed. The next section specifically highlights the various nanocomposite probes based on antifouling-nanomaterials for electrode modification containing carbon nanomaterials, noble metal nanoparticles, magnetic nanoparticles, polymer, and silicon-based materials in terms of nanoparticles, rods, or porous materials through optical or chemical strategies. We specially outline those nanoprobes that are capable of identification in complex media or those using new constructions/methods. Finally, the necessity and requirements for future advances in this emerging field are also presented, followed by opportunities and challenges.

Clinical Evaluation of a Novel Insulin Immunosensor

BACKGROUND

The estimation of available active insulin remains a limitation of automated insulin delivery systems. Currently, insulin pumps calculate active insulin using mathematical decay curves, while quantitative measurements of insulin would explicitly provide person-specific PK insulin dynamics to assess remaining active insulin more accurately, permitting more effective glucose control.

METHODS

We performed the first clinical evaluation of an insulin immunosensor chip, providing near real-time measurements of insulin levels. In this study, we sought to determine the accuracy of the novel insulin sensor and assess its therapeutic risk and benefit by presenting a new tool developed to indicate the potential therapeutic consequences arising from inaccurate insulin measurements.

RESULTS

Nine adult participants with type-1 diabetes completed the study. The change from baseline in immunosensor-measured insulin levels was compared with values obtained by standard enzyme-linked immunosorbant assay (ELISA) after preprandial injection of insulin. The point-of-care quantification of insulin levels revealed similar temporal trends as those from the laboratory insulin ELISA. The results showed that 70% of the paired immunosensor-reference values were concordant, which suggests that the patient could take action safely based on insulin concentration obtained by the novel sensor.

CONCLUSIONS

This proposed technology and preliminary feasibility evaluation show encouraging results for near real-time evaluation of insulin levels, with the potential to improve diabetes management. Real-time measurements of insulin provide person-specific insulin dynamics that could be used to make more informed decisions regarding insulin dosing, thus helping to prevent hypoglycemia and improve diabetes outcomes.

Development of a Novel Insulin Sensor for Clinical Decision-Making

BACKGROUND

Clinical decision support systems that incorporate information from frequent insulin measurements to enhance individualized diabetes management remain an unmet goal. The development of a disposable insulin strip for fast decentralized point-of-care detection replacing the current centralized lab-based methods used in clinical practice would be highly desirable to improve the establishment of individual insulin absorption patterns and algorithm modeling processes.

METHODS

We carried out the development and optimization of a novel decentralized disposable insulin electrochemical sensor focusing on obtaining high analytical and operational performance toward achieving a true point-of-care insulin testing device for clinical on-site application.

RESULTS

Our novel insulin immunosensor demonstrated an attractive performance and efficient user-friendly operation by providing high sensitivity capability to detect endogenous and analog insulin with a limit of detection of 30.2 pM (4.3 µiU/mL), rapid time-to-result, stability toward remote site application, and scalable low-cost fabrication with an estimated cost-of-goods for disposable consumables of below $5, capable of near real-time insulin detection in a microliter (≤10 µL) sample droplet of undiluted serum within 30 minutes.

CONCLUSIONS

The results obtained in the optimization and characterization of our novel insulin sensor illustrate its suitability for its potential application in remote clinical environments for frequent insulin monitoring. Future work will test the insulin sensor in a clinical research setting to assess its efficacy in individuals with type 1 diabetes.

Lab under the Skin: Microneedle Based Wearable Devices

While the current smartwatches and cellphones can readily track mobility and vital signs, a new generation of wearable devices is rapidly developing to enable users to monitor their health parameters at the molecular level. Within this emerging class of wearables, microneedle-based transdermal sensors are in a prime position to play a key role in synergizing the significant advantages of dermal interstitial fluid (ISF) as a rich source of clinical indicators and painless skin pricking to allow the collection of real-time diagnostic information. While initial efforts of microneedle sensing focused on ISF extraction coupled with either on-chip analysis or off-chip instrumentation, the latest trend has been oriented toward assembling electrochemical biosensors on the tip of microneedles to allow direct continuous chemical measurements. In this context, significant advances have recently been made in exploiting microneedle-based devices for real-time monitoring of various metabolites, electrolytes, and therapeutics and toward the simultaneous multiplexed detection of key chemical markers; yet, there are several grand challenges that still exist. In this review, we outline current progress, recent trends, and new capabilities of microneedle-empowered sensors, along with the current unmet challenges and a future roadmap toward transforming the latest innovations in the field to commercial products.

Ratiometric Antifouling Electrochemical Biosensors Based on Multifunctional Peptides and MXene Loaded with Au Nanoparticles and Methylene Blue

A universal strategy for the construction of ratiometric antifouling electrochemical biosensors was developed based on multifunctional peptides and 2D nanomaterial MXene loaded with gold nanoparticles (AuNPs) and methylene blue (MB). The nanocomposite of MXene loaded with AuNPs and MB (MXene-Au-MB) exhibited excellent conductivity, where the AuNPs were able to capture biomolecules containing sulfhydryl terminus, and the MB molecules were used to generate electrochemical signal. The MXene-Au-MB was fixed on the electrode surface by Nafion, and the anchored peptide captured the electrochemical signal probe carboxyl-modified ferrocene (Fc) to construct an electrochemical biosensor. The multifunctional peptide containing the anchoring, antifouling, and recognizing sequences endowed the sensing surface not only the assaying function but also the capability to resist nonspecific adsorption from complex samples. In the biosensing system, with the increase in the target concentration, the electrochemical signal of MB remained constant, whereas the electrochemical signal of Fc gradually decreased, and the ratiometric detection strategy greatly improved the accuracy of the biosensor. In the presence of a model target prostate-specific antigen (PSA), the recognizing sequence was recognized and cleaved, and the ratiometric signal of Fc and MB indicated the concentration of PSA accurately and sensitively, with a detection range from 5 pg/mL to 10 ng/mL and a limit of detection of 0.83 pg/mL. Electrochemical biosensors based on the MXene-Au-MB and multifunctional peptides possessed high selectivity, accuracy, and sensitivity even in real complex biological samples because of the excellent antifouling ability of the peptide. More importantly, the assaying of other targets can be easily realized with a similar biosensing strategy by changing the recognition sequence of the multifunctional peptide, and the detection of thrombin (TB) has also been achieved in this work.

A review of biomarkers in the context of type 1 diabetes: Biological sensing for enhanced glucose control

As wearable healthcare monitoring systems advance, there is immense potential for biological sensing to enhance the management of type 1 diabetes (T1D). The aim of this work is to describe the ongoing development of biomarker analytes in the context of T1D. Technological advances in transdermal biosensing offer remarkable opportunities to move from research laboratories to clinical point-of-care applications. In this review, a range of analytes, including glucose, insulin, glucagon, cortisol, lactate, epinephrine, and alcohol, as well as ketones such as beta-hydroxybutyrate, will be evaluated to determine the current status and research direction of those analytes specifically relevant to T1D management, using both in-vitro and on-body detection. Understanding state-of-the-art developments in biosensing technologies will aid in bridging the gap from bench-to-clinic T1D analyte measurement advancement.

Strategies for Biomolecular Analysis and Continuous Physiological Monitoring

Portable devices capable of rapid disease detection and health monitoring are crucial to decentralizing diagnostics from clinical laboratories to the patient point-of-need. Although technologies have been developed targeting this challenge, many require the use of reporter molecules or reagents that complicate the automation and autonomy of sensors. New work in the field has targeted reagentless approaches to enable breakthroughs that will allow personalized monitoring of a wide range of biomarkers on demand. This Perspective focuses on the ability of reagentless platforms to revolutionize the field of sensing by allowing rapid and real-time analysis in resource-poor settings. First, we will highlight advantages of reagentless sensing techniques, specifically electrochemical detection strategies. Advances in this field, including the development of wearable and in situ sensors capable of real-time monitoring of biomarkers such as nucleic acids, proteins, viral particles, bacteria, therapeutic agents, and metabolites, will be discussed. Reagentless platforms which allow for wash-free, calibration free-detection with increased dynamic range are highlighted as a key technological advance for autonomous sensing applications. Furthermore, we will highlight remaining challenges which must be overcome to enable widespread use of reagentless devices. Finally, future prospects and potential breakthroughs in precision medicine that will arise as a result of further development of reagentless sensing approaches are discussed.

The impact of antifouling layers in fabricating bioactive surfaces

Bioactive surfaces modified with functional peptides are critical for both fundamental research and practical application of implant materials and tissue repair. However, when bioactive molecules are tethered on biomaterial surfaces, their functions can be compromised due to unwanted fouling (mainly nonspecific protein adsorption and cell adhesion). In recent years, researchers have continuously studied antifouling strategies to obtain low background noise and effectively present the function of bioactive molecules. In this review, we describe several commonly used antifouling strategies and analyzed their advantages and drawbacks. Among these strategies, antifouling molecules are widely used to construct the antifouling layer of various bioactive surfaces. Subsequently, we summarize various structures of antifouling molecules and their surface grafting methods and characteristics. Application of these functionalized surfaces in microarray, biosensors, and implants are also introduced. Finally, we discuss the primary challenges associated with antifouling layers in fabricating bioactive surfaces and provide prospects for the future development of this field. STATEMENT OF SIGNIFICANCE: The nonspecific protein adsorption and cell adhesion will cause unwanted background "noise" on the surface of biological materials and detecting devices and compromise the performance of functional molecules and, therefore, impair the performance of materials and the sensitivity of devices. In addition, the selection of antifouling surfaces with proper chain length and high grafting density is also of great importance and requires further studies. Otherwise, the surface-tethered bioactive molecules may not function in their optimal status or even fail to display their functions. Based on these two critical issues, we summarize antifouling molecules with different structures, variable grafting methods, and diverse applications in biomaterials and biomedical devices reported in literature. Overall, we expect to shed some light on choosing the appropriate antifouling molecules in fabricating bioactive surfaces.

Electrochemical sensing interfaces based on hierarchically architectured zwitterionic peptides for ultralow fouling detection of alpha fetoprotein in serum

Herein, an electrochemical sensing platform based on zwitterionic peptide with a hierarchical structure was constructed for ultralow fouling and highly sensitive protein quantification. Through the combination of CPPPPEKEKEKEK and CPPPPEKEKEK peptides, hierarchical antifouling peptide brushes were formed and exhibited excellent antifouling property, which can be further modified with alpha fetoprotein (AFP) aptamer to achieve highly sensitive detection of AFP. The hierarchical peptide brush-based sensor system achieved an AFP quantification range from 1.0 fg mL^-1 to 1.0 ng mL^-1, with a very low limit of detection as low as 0.59 fg mL^-1. In addition, due to the superior antifouling property of the newly designed hierarchical peptide brushes, the electrochemical biosensor supported the quantification of AFP in solutions with a high concentration of nonspecific proteins without sacrifice in sensitivity. It is worth noting that the constructed antifouling biosensor ensured quantitative recruitment of AFP in clinical serum samples with acceptable accuracy when compared with the commonly used method in the hospital. The strategy of constructing sensing interfaces based on designed hierarchical peptide brushes provided an effective way to develop biosensors with both excellent antifouling capability and high sensitivity.

Reusable Cyclodextrin-Based Electrochemical Platform for Detection of trans-Resveratrol

A reusable sensor architecture, through the combination of self-assembled monolayers and cyclodextrin supramolecular interactions, is demonstrated for class recognition of hydrophobic analytes demonstrated with trans-resveratrol. The reloadable sensor is based on reversible immobilization of α-cyclodextrin on polyethylene glycol surface. α-cyclodextrins complexes with polyethylene glycols and causes the polymer chains to change their surface configuration. The reproducibility and stability of the sur-face, in the detection of nanomolar concentrations of trans-resveratrol, can be demonstrated by electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and Attenuated total reflectance-Fourier transform infrared spectroscopy. We propose that during sensor operation, α-cyclodextrin decouples from the poly-ethylene glycol surface to complex with trans-resveratrol in solution, and after use, the surface regeneration is conducted with a simple α-cyclodextrin soak. To test the nonspecific response, the sensor was also tested with trans-resveratrol spiked human urine.

Scaling-up medical technologies using flexographic printing

Medical technologies, such as point-of-care devices and biological and chemical assays which rely on functional materials deposited on top of substrates, are in great demand due to an increase in the prevalence of diseases worldwide. A significant number of these medical technologies are still in their infancy with respect to commercialization because of the high cost, material and complexity of the conventionally available fabrication techniques. As a result, medical technologies, in broad terms, require low cost and mass production fabrication methods in order to overcome the commercialization challenges. Recently, researchers have explored the flexographic printing technique which is widely employed for food packaging and newspaper production. This technique has proved cost-effective, facile, rapid and industrially compatible fabrication technique of functional materials for various applications. In this review, we provide an account of the attempts of flexographic printing made to scale up functional materials on surfaces for biomedical applications. Firstly, we offer justification for demanding high-throughput fabrication techniques. We then present the facile working principle of the flexographic printing and its use in different medical applications, for example chronic disease monitoring devices, colorimetric sensors, electrochemical sensors, assays and drugs. Finally, we discuss challenges of the fabrication technique. The main purpose of this review is to give insights into the usefulness of flexographic printing to the health care industry.

Printed Electrodes in Microfluidic Arrays for Cancer Biomarker Protein Detection

Medical diagnostics is trending towards a more personalized future approach in which multiple tests can be digitized into patient records. In cancer diagnostics, patients can be tested for individual protein and genomic biomarkers that detect cancers at very early stages and also be used to monitor cancer progression or remission during therapy. These data can then be incorporated into patient records that could be easily accessed on a cell phone by a health care professional or the patients themselves on demand. Data on protein biomarkers have a large potential to be measured in point-of-care devices, particularly diagnostic panels that could provide a continually updated, personalized record of a disease like cancer. Electrochemical immunoassays have been popular among protein detection methods due to their inherent high sensitivity and ease of coupling with screen-printed and inkjet-printed electrodes. Integrated chips featuring these kinds of electrodes can be built at low cost and designed for ease of automation. Enzyme-linked immunosorbent assay (ELISA) features are adopted in most of these ultrasensitive detection systems, with microfluidics allowing easy manipulation and good fluid dynamics to deliver reagents and detect the desired proteins. Several of these ultrasensitive systems have detected biomarker panels ranging from four to eight proteins, which in many cases when a specific cancer is suspected may be sufficient. However, a grand challenge lies in engineering microfluidic-printed electrode devices for the simultaneous detection of larger protein panels (e.g., 50-100) that could be used to test for many types of cancers, as well as other diseases for truly personalized care.

Design and fabrication of a smart sensor using in silico epitope mapping and electro-responsive imprinted polymer nanoparticles for determination of insulin levels in human plasma

A robust and highly specific sensor based on electroactive molecularly imprinted polymer nanoparticles (nanoMIP) was developed. The nanoMIP tagged with a redox probe, combines both recognition and reporting capabilities. The developed nanoMIP replaces enzyme-mediator pairs used in traditional biosensors thus, offering enhanced molecular recognition for insulin, improving performance in complex biological samples, and yielding high stability. Also, most of existing sensors show poor performance after storage. To improve costs of the logistics and avoid the need of cold storage in the chain supply, we developed an alternative to biorecognition system that relies on nanoMIP. NanoMIP were computationally designed using "in-silico" insulin epitope mapping and synthesized by solid phase polymerisation. The characterisation of the polymer nanoparticles was performed by transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier-transform Infrared (FT-IR) and surface plasmon resonance (SPR). The electrochemical sensor was developed by chemical immobilisation of the nanoMIP on screen printed platinum electrodes. The insulin sensor displayed satisfactory performances and reproducible results (RSD = 4.2%; n = 30) using differential pulse voltammetry (DPV) in the clinically relevant concentration range from 50 to 2000 pM. The developed nanoMIP offers the advantage of large number of specific recognition sites with tailored geometry, as the resultant, the sensor showed high sensitivity and selectivity to insulin with a limit of detection (LOD) of 26 and 81 fM in buffer and human plasma, respectively, confirming the practical application for point of care monitoring. Moreover, the nanoMIP showed adequate storage stability of 168 days, demonstrating the robustness of sensor for several rounds of insulin analysis.

A Probeless Capacitive Biosensor for Direct Detection of Amyloid Beta 1-42 in Human Serum Based on an Interdigitated Chain-Shaped Electrode

Amyloid beta (aβ) 1-42, a peptide that is 1-42 amino acids long, is a major component of senile plaques in the brains of patients with Alzheimer's disease. Aβ detection has become an essential antecedence to predict the declining mental abilities of patients. In this paper, a probeless capacitive biosensor for the non-Faradaic detection of aβ 1-42 peptide was developed by immobilizing a specific anti-aβ antibody onto a self-assembled monolayer functionalized interdigitated chain-shaped electrode (anti-aβ/SAM/ICE). The novelty and difference of this article from previous studies is the direct detection of aβ peptide with no redox probe ((Fe(CN)6)3-/4-), which can avoid the denaturation of the protein caused by the metallization (binding of aβ to metal ion Fe which is presented in the redox couple). The direct detection of aβ with no redox probe is performed by non-Faradaic capacitive measurement, which is greatly different from the Faradaic measurement of the charge transfer resistance of the redox probe. The detection of various aβ 1-42 peptide concentrations in human serum (HS) was performed by measuring the relative change in electrode interfacial capacitance due to the specific antibody-aβ binding. Capacitance change in the anti-aβ/SAM/ICE biosensor showed a linear detection range between 10 pg mL-1 and 104 pg mL-1, and a detection limit of 7.5 pg mL-1 in HS, which was much lower than the limit of detection for CSF aβ 1-42 (~500 pg mL-1) and other biosensors. The small dissociation constant Kd of the antibody-antigen interaction was also found to be 0.016 nM in HS, indicating the high binding affinity of the anti-aβ/SAM/ICE biosensor in the recognizing of aβ 1-42. Thus, the developed sensor can be used for label-free and direct measurement of aβ 1-42 peptide and for point-of-care diagnosis of Alzheimer's disease without redox probe.

Functional Molecular Interfaces for Impedance-Based Diagnostics

In seeking to develop and optimize reagentless electroanalytical assays, a consideration of the transducing interface features lies key to any subsequent sensitivity and selectivity. This review briefly summarizes some of the most commonly used receptive interfaces that have been employed within the development of impedimetric molecular sensors. We discuss the use of high surface area carbon, nanoparticles, and a range of bioreceptors that can subsequently be integrated. The review spans the most commonly utilized biorecognition elements, such as antibodies, antibody fragments, aptamers, and nucleic acids, and touches on some novel emerging alternatives such as nanofragments, molecularly imprinted polymers, and bacteriophages. Reference is made to the immobilization chemistries available along with a consideration of both optimal packing density and recognition probe orientation. We also discuss assay-relevant mechanistic details and applications in real sample analysis.

Superhydrophilicity and strong salt-affinity: Zwitterionic polymer grafted surfaces with significant potentials particularly in biological systems

In recent years, zwitterionic polymers have been frequently reported to modify various surfaces to enhance hydrophilicity, antifouling and antibacterial properties, which show significant potentials particularly in biological systems. This review focuses on the fabrication, properties and various applications of zwitterionic polymer grafted surfaces. The "graft-from" and "graft-to" strategies, surface grafting copolymerization and post zwitterionization methods were adopted to graft lots type of the zwitterionic polymers on different inorganic/organic surfaces. The inherent hydrophilicity and salt affinity of the zwitterionic polymers endow the modified surfaces with antifouling, antibacterial and lubricating properties, thus the obtained zwitterionic surfaces show potential applications in biosystems. The zwitterionic polymer grafted membranes or stationary phases can effectively separate plasma, water/oil, ions, biomolecules and polar substrates. The nanomedicines with zwitterionic polymer shells have "stealth" effect in the delivery of encapsulated drugs, siRNA or therapeutic proteins. Moreover, the zwitterionic surfaces can be utilized as wound dressing, self-healing or oil extraction materials. The zwitterionic surfaces are expected as excellent support materials for biosensors, they are facing the severe challenges in the surface protection of marine facilities, and the dense ion pair layers may take unexpected role in shielding the grafted surfaces from strong electromagnetic field.

Functional zwitterionic biomaterials for administration of insulin

This review summarizes the structures and biomedical applications of zwitterionic biomaterials in the administration of insulin.

Perspectives on and Precautions for the Uses of Electric Spectroscopic Methods in Label-free Biosensing Applications

Label-free approaches for molecular diagnostic applications are appealing because of their inherent point-of-care advantages. Nonetheless, technical challenges impose a limit on the use of these methods as will be discussed in this paper. Electrochemical spectroscopic methods, such as impedance and impedance-derived methods, are highly effective in the development of label-free diagnostic assays, but they require careful control of the dynamics of the sensing interface. We herein report the strength and challenges of the current methodologies associated with the applications of impedance and impedance-derived methods by focusing on their principles of operation. We demonstrate that the uses of their potentialities are not based on the know-how of these methods, but on how to combine the spectroscopic features with the required chemical design for the associated sensing interfaces. Predominantly, we illustrate how to use the resistive and capacitive terms of the interface to improve its sensitivity to the target. For instance, with the proper signal amplification strategy, limitations related to target-to-receptor size ratio can be overcome. The target-to-receptor ratio is one of the difficulties that we use as an example to illustrate how the sensing of an electric signal can be improved by controlling the properties of the interface on the nanometer scale.

Low fouling and ultrasensitive electrochemical immunosensors with dual assay methods based on Fe3O4 magnetic nanoparticles

Low fouling electrochemical immunosensors with both "signal-off" and "signal-on" analytical methods were developed for the highly sensitive and efficient detection of cancer antigen 15-3 (CA 15-3) in human serum samples. The antifouling sensing interfaces were constructed by assembling multifunctional polyethylene glycol on gold electrodes, followed by covalent conjugation with CA 15-3 antibody. Pure antigens and Fe₃O₄@Ag will competitively bind to the immobilized antibody on the electrode. Fe₃O₄ magnetic nanoparticles attached to the working electrode and collected by a magnetic electrode were treated via electrochemical conversion to generate electroactive Prussian blue as a signal readout. Therefore, these two signals measured independently were complementary, and this design allowed one to choose the assay method according to real situations so as to ensure accuracy of the immunosensor. Moreover, owing to its good antifouling property, the immunosensor was capable of detecting CA 15-3 even in complex human serum samples, demonstrating potential application in quantitative analysis of real patient serum samples.

Recent Advances on Electrochemical Sensors for the Detection of Organic Disinfection Byproducts in Water

Irreversible organ damage or even death frequently occurs when humans or animals unknowingly drink contaminated water. Therefore, in many countries drinking water is disinfected to ensure removal of harmful pathogens from drinking water. If upstream water treatment prior to disinfection is not adequate, disinfection byproducts (DBPs) can be formed. DBPs can exist as wide variety of compounds, but up until now, only several typical compounds have drinking water standards attributed to them. However, it is apparent that the range of DBPs present in water can comprise hundreds of compounds, some of which are at high enough concentrations to be toxic or potentially carcinogenic. Hence, it becomes increasingly significant and urgent to develop an accessible, affordable, and durable sensing platform for a broader range and more sensitive detection of DBPs. Compared with well-established laboratory detection techniques, electrochemical sensing has been identified as a promising alternative that will provide rapid, affordable, and sensitive DBP monitoring in remote water sources. Therefore, this Review covers current state-of-the-art development (within the past decade) in electrochemical sensing to detect organic DBPs in water, which covered three major aspects: (1) recognition mechanism, (2) electrodes with signal amplification, and (3) signal read-out techniques. Moreover, comprehensive quality assessments on electrochemical biosensors, including linear detection range, limit of detection (LoD) and recovery, have also been summarized.

Dopamine Binding and Analysis in Undiluted Human Serum and Blood by the RNA-Aptamer Electrode

Specific analysis of such neurotransmitters as dopamine by the aptamer electrodes in biological fluids is detrimentally affected by nonspecific adsorption of media, particularly pronounced at positive charges of the electrode surface at which dopamine oxidizes. Here, we show that dopamine analysis at the RNA-aptamer/cysteamine-modified electrodes is strongly inhibited in undiluted human serum and blood due to nonspecific interfacial adsorption of serum and blood components. We demonstrate that nonspecific adsorption of serum proteins (but not of blood components) could be minimized when analysis is performed in a flow and injections of serum samples are followed by washing steps in a phosphate buffer solution (PBS) carrier. Under those conditions, the dopamine-aptamer binding affinity in whole human serum of (1.9 ± 0.3) × 10⁴ M^-1 s^-1 was comparable to the (3.7 ± 0.3) × 10⁴ M^-1 s^-1 found in PBS, and the dopamine oxidation signal linearly depended on the dopamine concentration, providing a sensitivity of analysis of 73 ± 3 nA μM^-1 cm^-2 and a LOD of 114 ± 8 nM. The flow-injection apatmer-electrode system was used for direct analysis of basal levels of dopamine in undiluted human serum samples, without using any physical separators (membranes) or filtration procedures. The results suggest a simple strategy for combatting biosurface fouling, otherwise most pronounced at positive electrode potentials used for dopamine detection, and assist in designing more efficient antifouling strategies for biomedical applications.

Low fouling electrochemical sensing in complex biological media by using the ionic liquid-doped conducting polymer PEDOT: application to voltammetric determination of dopamine

An electrochemical sensor that can resist biofouling even when operated in complex biological medium is developed for the determination of dopamine. It is based on the use of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) that is doped with the water insoluble ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. A glassy carbon electrode modified with PEDOT/IL is shown to enable accurate determination of dopamine, as a model analyte in the presence of high concentrations of proteins, and resist biological fouling even in native serum. It exhibited a low limit of detection of 33 nM for the detection of dopamine, with a wide linear range from 0.2 to 328 μM (at 0.2 V vs. saturated calomel electrode). The PEDOT/IL modified glassy carbon electrode has a porous microstructure, high electrical conductivity and good stability. The sensor can be used to quantify dopamine in human urine samples with satisfying accuracy. Graphical abstract An antifouling electrochemical sensor capable of detecting target in complex biological samples was developed based on the use of a conducting polymer (PEDOT) that was doped with a water insoluble ionic liquid.

Towards Point-of-Care Insulin Detection

Good glucose management through an insulin dose regime based on the metabolism of glucose helps millions of people worldwide manage their diabetes. Since Banting and Best extracted insulin, glucose management has improved due to the introduction of insulin analogues that act from 30 minutes to 28 days, improved insulin dose regimes, and portable glucose meters, with a current focus on alternative sampling sites that are less invasive. However, a piece of the puzzle is still missing-the ability to measure insulin directly in a Point-of-Care device. The ability to measure both glucose and insulin concurrently will enable better glucose control by providing an improved estimate for insulin sensitivity, minimizing variability in control, and maximizing safety from hypoglycaemia. However, direct detection of free insulin has provided a challenge due to the size of the molecule, the low concentration of insulin in blood, and the selectivity against interferants in blood. This review summarizes current insulin detection methods from immunoassays to analytical chemistry, and sensors. We also discuss the challenges and potential of each of the methods towards Point-of-Care insulin detection.

Interfacial capacitance immunosensing using interdigitated electrodes: the effect of insulation/immobilization chemistry

With the aim of improving the reproducibility of capacitive immunosensors, we performed a comparative study of four different insulating/immobilization chemistries.

Electrochemical Biosensing of Algal Toxins in Water: The Current State-of-the-Art

Due to increasing stringency of water legislation and extreme consequences that failure to detect some contaminants in water can involve, there has been a strong interest in developing electrochemical biosensors for algal toxin detection during the past decade, evidenced by literature increasing from 2 journal papers pre-2009 to 24 between 2009 and 2018. In this context, this review has summarized recent progress of successful algal toxin detection in water using electrochemical biosensing techniques. Satisfactory detection recoveries using real environmental water samples and good sensor repeatability and reproducibility have been achieved, along with some excellent limit-of-detection (LOD) reported. Recent electrochemical biosensor literature in algal toxin detection is compared and discussed to cover three major design components: (1) biorecognition elements, (2) electrochemical read-out techniques, and (3) sensor electrodes and signal amplification strategy. The recent development of electrochemical biosensors has provided one more step further toward quick in situ detection of algal toxins in the contamination point of the water source. In the end, we have also critically reviewed the current challenges and research opportunities regarding electrochemical biosensors for algal toxin detection that need to be addressed before they attain commercial viability.

Smart Cell Culture Systems: Integration of Sensors and Actuators into Microphysiological Systems

Technological advances in microfabrication techniques in combination with organotypic cell and tissue models have enabled the realization of microphysiological systems capable of recapitulating aspects of human physiology in vitro with great fidelity. Concurrently, a number of analysis techniques has been developed to probe and characterize these model systems. However, many assays are still performed off-line, which severely compromises the possibility of obtaining real-time information from the samples under examination, and which also limits the use of these platforms in high-throughput analysis. In this review, we focus on sensing and actuation schemes that have already been established or offer great potential to provide in situ detection or manipulation of relevant cell or tissue samples in microphysiological platforms. We will first describe methods that can be integrated in a straightforward way and that offer potential multiplexing and/or parallelization of sensing and actuation functions. These methods include electrical impedance spectroscopy, electrochemical biosensors, and the use of surface acoustic waves for manipulation and analysis of cells, tissue, and multicellular organisms. In the second part, we will describe two sensor approaches based on surface-plasmon resonance and mechanical resonators that have recently provided new characterization features for biological samples, although technological limitations for use in high-throughput applications still exist.

Antifouling and ultrasensitive biosensing interface based on self-assembled peptide and aptamer on macroporous gold for electrochemical detection of immunoglobulin E in serum

Accurate detection of protein biomarkers in complex media remains a challenge due to severe nonspecific adsorption and biofouling, and sensing interfaces that combine the high sensitivity and antifouling ability are highly desirable. Herein, an antifouling sensing interface capable of sensitively assaying immunoglobulin E (IgE) in biological samples was constructed. The sensing interface was fabricated through the self-assembly of a zwitterionic peptide and the IgE aptamer onto a macroporous Au substrate, which was electrochemically fabricated with the aid of multilayer polystyrene nanospheres self-assembled on glassy carbon electrode. Due to the huge surface area arising from porous morphology and high specificity of aptamer, the developed electrochemical biosensor exhibits ultrahigh sensitivity and selectivity towards IgE, with the linear range of 0.1-10 pg mL^-1, and a very low limit of detection down to 42 fg mL^-1. Interestingly, owing to the presence of the zwitterionic peptide, the biosensing interface can satisfyingly reduce the nonspecific adsorption and fouling effect. Consequently, the biosensor was successfully applied to detect IgE in complex biological samples, indicating great promise of this peptide-based sensing interface for antifouling assays. Graphical abstract ᅟ.

Electrochemical and surface plasmon insulin assays on clinical samples

Diabetes is a complex immune disorder that requires extensive medical care beyond glycemic control. Recently, the prevalence of diabetes, particularly type 1 diabetes (T1D), has significantly increased from 5% to 10%, and this has affected the health-associated complication incidences in children and adults. The 2012 statistics by the American Diabetes Association reported that 29.1 million Americans (9.3% of the population) had diabetes, and 86 million Americans (age ≥20 years, an increase from 79 million in 2010) had prediabetes. Personalized glucometers allow diabetes management by easy monitoring of the high millimolar blood glucose levels. In contrast, non-glucose diabetes biomarkers, which have gained considerable attention for early prediction and provide insights about diabetes metabolic pathways, are difficult to measure because of their ultra-low levels in blood. Similarly, insulin pumps, sensors, and insulin monitoring systems are of considerable biomedical significance due to their ever-increasing need for managing diabetic, prediabetic, and pancreatic disorders. Our laboratory focuses on developing electrochemical immunosensors and surface plasmon microarrays for minimally invasive insulin measurements in clinical sample matrices. By utilizing antibodies or aptamers as the insulin-selective biorecognition elements in combination with nanomaterials, we demonstrated a series of selective and clinically sensitive electrochemical and surface plasmon immunoassays. This review provides an overview of different electrochemical and surface plasmon immunoassays for insulin. Considering the paramount importance of diabetes diagnosis, treatment, and management and insulin pumps and monitoring devices with focus on both T1D (insulin-deficient condition) and type 2 diabetes (insulin-resistant condition), this review on insulin bioassays is timely and significant.

Surface-Enhanced Raman Spectroscopy-Based Label-Free Insulin Detection at Physiological Concentrations for Analysis of Islet Performance

Label-free optical detection of insulin would allow in vitro assessment of pancreatic cell functions in their natural state and expedite diabetes-related clinical research and treatment; however, no existing method has met these criteria at physiological concentrations. Using spatially uniform 3D gold-nanoparticle sensors, we have demonstrated surface-enhanced Raman sensing of insulin in the secretions from human pancreatic islets under low and high glucose environments without the use of labels such as antibodies or aptamers. Label-free measurements of the islet secretions showed excellent correlation among the ambient glucose levels, secreted insulin concentrations, and measured Raman-emission intensities. When excited at 785 nm, plasmonic hotspots of the densely arranged 3D gold-nanoparticle pillars as well as strong interaction between sulfide linkages of the insulin molecules and the gold nanoparticles produced highly sensitive and reliable insulin measurements down to 100 pM. The sensors exhibited a dynamic range of 100 pM to 50 nM with an estimated detection limit of 35 pM, which covers the reported concentration range of insulin observed in pancreatic cell secretions. The sensitivity of this approach is approximately 4 orders of magnitude greater than previously reported results using label-free optical approaches, and it is much more cost-effective than immunoassay-based insulin detection widely used in clinics and laboratories. These promising results may open up new opportunities for insulin sensing in research and clinical applications.

Addendum: Plasmonic nanosensors with inverse sensitivity by means of enzyme-guided crystal growth

This corrects the article DOI: 10.1038/nmat3337.

Mixed Self-Assembly of Polyethylene Glycol and Aptamer on Polydopamine Surface for Highly Sensitive and Low-Fouling Detection of Adenosine Triphosphate in Complex Media

Detection of disease biomarkers within complex biological media is a substantial outstanding challenge because of severe biofouling and nonspecific adsorptions. Herein, a reliable strategy for sensitive and low-fouling detection of a biomarker, adenosine triphosphate (ATP) in biological samples was developed through the formation of a mixed self-assembled sensing interface, which was constructed by simultaneously self-assembling polyethylene glycol (PEG) and ATP aptamer onto the self-polymerized polydopamine-modified electrode surface. The developed aptasensor exhibited high selectivity and sensitivity toward the detection of ATP, and the linear range was 0.1-1000 pM, with a detection limit down to 0.1 pM. Moreover, owing to the presence of PEG within the sensing interface, the aptasensor was capable of sensing ATP in complex biological media such as human plasma with significantly reduced nonspecific adsorption effect. Assaying ATP in real biological samples including breast cancer cell lysates further proved the feasibility of this biosensor for practical application.

Enhancing Glycemic Control via Detection of Insulin Using Electrochemical Impedance Spectroscopy

BACKGROUND

Currently, glycemic management for individuals with diabetes mellitus involves monitoring glucose only, which is insufficient as glucose metabolism involves other biomarkers such as insulin. Monitoring additional biomarkers alongside glucose has been proposed to improve glycemic control. In this work, the development of a rapid and label-free insulin biosensor with high sensitivity and accuracy is presented. The insulin sensor prototype also serves as a prior study for a multimarker sensing platform technology that can further improve glycemic control in the future.

METHODS

Electrochemical impedance spectroscopy was used to identify an optimal frequency specific to insulin detection on a gold disk electrode with insulin antibody immobilized, which was accomplished by conjugating the primary amines of insulin antibody to the carboxylic bond of the self-assembling monolayer on the gold surface. After blocking with ethanolamine, the insulin physiological concentration gradient was tested. The imaginary impedance was correlated to insulin concentration and the results were compared with standard equivalent circuit analysis and correlation of charge transfer resistance to target concentration.

RESULTS

The optimal frequency of insulin is 810.5 Hz, which is characterized by having the highest sensitivity and sufficient specificity. The lower limit of detection was 2.26 [Formula: see text] which is comparable to a standard and better than traditional approaches.

CONCLUSION

An insulin biosensor prototype capable of detecting insulin in physiological range without complex data normalization was developed. This prototype will be the ground works of a multimarker platform sensor technology for future all-in-one glycemic management sensors.

Non-invasive, in vitro analysis of islet insulin production enabled by an optical porous silicon biosensor

A label-free porous silicon (pSi) based, optical biosensor, using both an antibody and aptamer bioreceptor motif has been developed for the detection of insulin. Two parallel biosensors were designed and optimised independently, based on each bioreceptor. Both bioreceptors were covalently attached to a thermally hydrosilylated pSi surface though amide coupling, with unreacted surface area rendered stable and low fouling by incorporation of PEG moieties. The insulin detection ability of each biosensor was determined using interferometric reflectance spectroscopy, using a range of different media both with and without serum. Sensing performance was compared in terms of response value, response time and limit of detection (LOD) for each platform. In order to demonstrate the capability of the best performing biosensor to detect insulin from real samples, an in vitro investigation with the aptamer-modified surface was performed. This biosensor was exposed to buffer conditioned by glucose-stimulated human islets, with the result showing a positive response and a high degree of selectivity towards insulin capture. The obtained results correlated well with the ELISA used in the clinic for assaying glucose-stimulated insulin release from donor islets. We anticipate that this type of sensor can be applied as a rapid point-of-use biosensor to assess the quality of donor islets in terms of their insulin production efficiency, prior to transplantation.

Surface Engineering of Nanoparticles to Create Synthetic Antibodies

Surface engineering of nanoparticles has recently emerged as a promising technique for synthetic molecular recognition of biological analytes. In particular, the use of synthetic heteropolymers adsorbed onto the surface of a nanoparticle can yield selective detection of a molecular target. Synthetic molecular recognition has unique advantages in leveraging the photostability, versatility, and exceptional chemical stability of nanomaterials. In particular, single-walled carbon nanotubes (SWNT) exhibit a large Stokes shift and near infrared emission for maximum biological sample transparency. Optical biosensors with high signal transduction and molecular specificity can be synthesized with amphiphilic heteropolymers grafted to SWNT, and discovered by high-throughput screening. Herein, we describe the development and the characterization of surface-engineered nanoparticles, or "synthetic antibodies," for protein detection.

Magnetic Optical Microarray Imager for Diagnosing Type of Diabetes in Clinical Blood Serum Samples

Due to rapidly rising rates of diabetes and prediabetic conditions worldwide and the associated lethal complications, it is imperative to devise new diagnostic tools that reliably and directly measure insulin levels in clinical samples. Herein, we report a simple and sensitive direct imaging of insulin levels in diabetic patient samples using a surface plasmon resonance microarray imager (SPRi). To enhance sensitivity, we utilized magnetic nanoparticles (MNPs) to capture insulin from serum samples either directly or via a capture antibody immobilized on MNPs. The insulin-captured nanoparticles were allowed to bind surface insulin-antibody for detection from pixel intensity increase using a charge coupled device (CCD) built-in with the SPRi. We have compared the analytical figures-of-merit of the SPRi immunoarray on detecting insulin prepared in various percentages of serum solutions. A four parameter logistic model was used to obtain the best fit of microarray responses with insulin concentration and indicated the cooperative binding of insulin-nanoparticle conjugates to surface antibody in both the buffer insulin and the serum insulin conjugates with MNPs. The cooperativity effect is attributed to the greater association of magnetic nanoparticle-bound insulin molecules with increasing concentration of insulin binding to surface antibody. This is the first report of an SPRi immunoarray to accomplish clinical diagnosis of diabetic and prediabetic conditions based on insulin levels with serum matrix effect analysis and comparison between direct and sandwich insulin assay formats.