Affinity and enzyme-based biosensors: recent advances and emerging applications in cell analysis and point-of-care testing

State-of-the-art in engineering small molecule biosensors and their applications in metabolic engineering

Genetically encoded biosensors are crucial for enhancing our understanding of how molecules regulate biological systems. Small molecule biosensors, in particular, help us understand the interaction between chemicals and biological processes. They also accelerate metabolic engineering by increasing screening throughput and eliminating the need for sample preparation through traditional chemical analysis. Additionally, they offer significantly higher spatial and temporal resolution in cellular analyte measurements. In this review, we discuss recent progress in in vivo biosensors and control systems-biosensor-based controllers-for metabolic engineering. We also specifically explore protein-based biosensors that utilize less commonly exploited signaling mechanisms, such as protein stability and induced degradation, compared to more prevalent transcription factor and allosteric regulation mechanism. We propose that these lesser-used mechanisms will be significant for engineering eukaryotic systems and slower-growing prokaryotic systems where protein turnover may facilitate more rapid and reliable measurement and regulation of the current cellular state. Lastly, we emphasize the utilization of cutting-edge and state-of-the-art techniques in the development of protein-based biosensors, achieved through rational design, directed evolution, and collaborative approaches.

Advances and perspectives in the analytical technology for small peptide hormones analysis: A glimpse to gonadorelin

In the last twenty years, we have witnessed an important evolution of bioanalytical approaches moving from conventional lab bench instrumentation to simpler, easy-to-use techniques to deliver analytical responses on-site, with reduced analysis times and costs. In this frame, affinity reagents production has also jointly advanced from natural receptors to biomimetic, abiotic receptors, animal-free produced. Among biomimetic ones, aptamers, and molecular imprinted polymers (MIPs) play a leading role. Herein, our motivation is to provide insights into the evolution of conventional and innovative analytical approaches based on chromatography, immunochemistry, and affinity sensing referred to as peptide hormones. Indeed, the analysis of peptide hormones represents a current challenge for biomedical, pharmaceutical, and anti-doping analysis. Specifically, as a paradigmatic example, we report the case of gonadorelin, a neuropeptide that in recent years has drawn a lot of attention as a therapeutic drug misused in doping practices during sports competitions.

Aptamer-based rapid diagnosis for point-of-care application

Aptasensors have attracted considerable interest and widespread application in point-of-care testing worldwide. One of the biggest challenges of a point-of-care (POC) is the reduction of treatment time compared to central facilities that diagnose and monitor the applications. Over the past decades, biosensors have been introduced that offer more reliable, cost-effective, and accurate detection methods. Aptamer-based biosensors have unprecedented advantages over biosensors that use natural receptors such as antibodies and enzymes. In the current epidemic, point-of-care testing (POCT) is advantageous because it is easy to use, more accessible, faster to detect, and has high accuracy and sensitivity, reducing the burden of testing on healthcare systems. POCT is beneficial for daily epidemic control as well as early detection and treatment. This review provides detailed information on the various design strategies and virus detection methods using aptamer-based sensors. In addition, we discussed the importance of different aptamers and their detection principles. Aptasensors with higher sensitivity, specificity, and flexibility are critically discussed to establish simple, cost-effective, and rapid detection methods. POC-based aptasensors' diagnostic applications are classified and summarised based on infectious and infectious diseases. Finally, the design factors to be considered are outlined to meet the future of rapid POC-based sensors.

Optical sensors based on electrospun membranes – principles, applications, and prospects for chemistry and biology

Electrospun membranes are promising substrates for receptor layer immobilization in optical sensors. Either colorimetric, luminescence, or Raman scattering signal can be used to detect the analyte.

2D Materials-Based Aptamer Biosensors: Present Status and Way Forward

Current advances in constructing functional nanomaterials and elegantly designed nanostructures have opened up new possibilities for the fabrication of viable field biosensors. Two-dimensional materials (2DMs) have fascinated much attention due to their chemical, optical, physicochemical, and electronic properties. They are ultrathin nanomaterials with unique properties such as high surface-to-volume ratio, surface charge, shape, high anisotropy, and adjustable chemical functionality. 2DMs such as graphene-based 2D materials, Silicate clays, layered double hydroxides (LDHs), MXenes, transition metal dichalcogenides (TMDs), and transition metal oxides (TMOs) offer intensified physicochemical and biological functionality and have proven to be very promising candidates for biological applications and technologies. 2DMs have a multivalent structure that can easily bind to single-stranded DNA/RNA (aptamers) through covalent, non-covalent, hydrogen bond, and π-stacking interactions, whereas aptamers have a small size, excellent chemical stability, and low immunogenicity with high affinity and specificity. This review discussed the potential of various 2D material-based aptasensor for diagnostic applications, e.g., protein detection, environmental monitoring, pathogens detection, etc.

Point-of-use sweat biosensor to track the endocrine-inflammation relationship for chronic disease monitoring

AIM

The hypothalamic-pituitary-adrenal axis is involved in maintaining homeostasis by engaging with the parasympathetic nervous system. During the process of disease affliction, this relationship is disturbed and there is an imbalance driven response observed.

MATERIALS & METHODS

By monitoring the two key components involved in these pathways, cortisol and TNF-α, the manifestations of chronic stress on the body's homeostasis can be evaluated in a comprehensive manner. This work highlights the development of an electrochemical detection system for the two biomarkers through human sweat.

RESULTS

Limit of detection and dynamic ranges are 1 ng/ml, 1-200 ng/ml for cortisol and 1 pg/ml, 1-1000 pg/ml for TNF-α.

CONCLUSION

This wearable system is designed to be a point of use, chronic disease self-monitoring and management platform.

Skin-Integrated Wearable Systems and Implantable Biosensors: A Comprehensive Review

Biosensors devices have attracted the attention of many researchers across the world. They have the capability to solve a large number of analytical problems and challenges. They are future ubiquitous devices for disease diagnosis, monitoring, treatment and health management. This review presents an overview of the biosensors field, highlighting the current research and development of bio-integrated and implanted biosensors. These devices are micro- and nano-fabricated, according to numerous techniques that are adapted in order to offer a suitable mechanical match of the biosensor to the surrounding tissue, and therefore decrease the body’s biological response. For this, most of the skin-integrated and implanted biosensors use a polymer layer as a versatile and flexible structural support, combined with a functional/active material, to generate, transmit and process the obtained signal. A few challenging issues of implantable biosensor devices, as well as strategies to overcome them, are also discussed in this review, including biological response, power supply, and data communication.

Electrospun nanofibers: a promising horizon toward the detection and treatment of cancer

Due to the increase in the number of cancer patients, because of environmental parameters, high stress, low immunity, etc., there is an urgent need to develop cost-effective sensors for early targeted detection of cancerous cells with adequate selectivity and efficiency. Early disease diagnosis is important, as it is necessary to start treatments before disease progression. On the other hand, we need new, more efficient cancer treatment approaches with minimized side effects, more biocompatibility, and easy disposal. Nanobiotechnology is a field that can assist in developing new diagnostic and treatment approaches, specifically in fatal cancers. Herein, a study on the different applications of nanofibers in cancer detection as well as its treatment has been done. Here, a very brief survey on the main structure of biosensors and their different categories has been conducted and will precede the discussion of the study to serve as a reference and guide the reader's understanding.

Aptamer based nanobiosensors: Promising healthcare devices

Nanobiosensors based on aptamer are extensively being studied as potent analytical tools in clinical analysis. These biosensors provide high sensitivity, fast response, specificity and desired portability in addition to simplicity and decreased cost compared to conventional methods. The purpose of this manuscript is to provide readers with an overview of current advances about electrochemical, electrochemiluminescent and photoelectrochemical aptasensors from the sea of available literature. These are mainly used for determination of protein-based biomarkers, especially for cancer diagnosis. Here in we have given special emphasis on nanosize-based aptasensors which have been reported to show considerable improvement in the analytical performance.

Electrochemical Probes of Microbial Community Behavior

Advances in next-generation sequencing technology along with decreasing costs now allow the microbial population, or microbiome, of a location to be determined relatively quickly. This research reveals that microbial communities are more diverse and complex than ever imagined. New and specialized instrumentation is required to investigate, with high spatial and temporal resolution, the dynamic biochemical environment that is created by microbes, which allows them to exist in every corner of the Earth. This review describes how electrochemical probes and techniques are being used and optimized to learn about microbial communities. Described approaches include voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy, separation techniques coupled with electrochemical detection, and arrays of complementary metal-oxide-semiconductor circuits. Microbial communities also interact with and influence their surroundings; therefore, the review also includes a discussion of how electrochemical probes optimized for microbial analysis are utilized in healthcare diagnostics and environmental monitoring applications.

Carboxylic group riched graphene oxide based disposable electrochemical immunosensor for cancer biomarker detection

In this work, we have developed for the first time a carboxylic group riched graphene oxide based disposable electrochemical immunosensor for cancer biomarker detection using methylene blue (MB). The developed immunosensor is highly sensitive for detection of biomarker Mucin1 (MUC1) in human serum samples. Development of this disposable electrochemical immunosensor was premeditated by applying specific monoclonal antibodies against MUC1. In this method, we explored highly conductive surface of carboxylic group (-COOH-) rich graphene oxide (GO) on screen-printed carbon electrodes (SPCE). This modified GO-COOH-SPCE was employed for the detection of MUC1 protein based on the reaction with methylene blue (MB) redox probe using differential pulse voltammetry (DPV) technique. Developed immunosensor exhibited good detection range for MUC1 with excellent linearity (0.1 U/mL- 2 U/mL), with a limit of detection of 0.04 U/mL. Upon potential application of developed biosensor, good recoveries were recorded in the range of 96-96.67% with % R.S.D 4.2. Analytical performance of the developed immunosensor assures the applicability in clinical diagnostic applications.

Synthesis of Recombinant Mouse Crystallin Proteins and in Vitro Measurement of Their Refractivity

The eye lens is an organ that focuses light onto the retina and is reported to have a high refractive index in vertebrates. An analysis of refractivity was conducted using recombinant mouse Crystallin proteins produced in Escherichia coli (E. coli) compared with bovine serum albumin (BSA) and other commercially available proteins. Not only did we measure the refractivity but for one of the crystallins, Cryba1, we also confirmed that it responds uniquely to its environmental conditions. The crystallin showed high refractivity, as expected, and we confirmed that the electrical charge of the Cryba1 molecule influences its refractivity.

Lactate Detection in Tumor Cell Cultures Using Organic Transistor Circuits

A biosensing platform based on an organic transistor circuit for metabolite detection in highly complex biological media is introduced. The sensor circuit provides inherent background subtraction allowing for highly specific, sensitive lactate detection in tumor cell cultures. The proposed sensing platform paves the way toward rapid, label-free, and cost-effective clinically relevant in vitro diagnostic tools.

Biosensing Probe for Quality Control Monitoring of the Structural Integrity of Therapeutic Antibodies

Ideal quality control of therapeutic antibodies involves analytical techniques with high-sensitivity, high-resolution, and high-throughput performance. Few technologies that assess the physicochemical heterogeneity of antibodies, however, meet all the required demands. We developed a biosensing method for the quality control of therapeutic antibodies based on an artificial protein, AF.2A1, which discriminates between the native and the non-native three-dimensional structures of immunoglobulin G (IgG). AF.2A1 specifically recognized non-native IgG spiked into a solution of native IgG, thereby making it possible to detect contamination by a small amount of non-native IgG, which is difficult using conventional size-based separation or spectroscopic techniques. Using AF.2A1 as an analytical probe, we determined the concentration of non-native IgG formed under various pH conditions. The probe was also applicable to accelerated tests of the long-term stability of a therapeutic antibody, allowing monitoring of the formation of non-native IgG at elevated temperatures and extended periods of storage. AF.2A1, a proteinous probe, can be combined with established methods or devices to achieve high-throughput assays and provides the potential for probe-based biosensing for the quality control of therapeutic antibodies.

Organic Transistor Arrays Integrated with Finger-Powered Microfluidics for Multianalyte Saliva Testing

A compact multianalyte biosensing platform is reported, composed of an organic electrochemical transistor (OECT) microarray integrated with a pumpless "finger-powered" microfluidic, for quantitative screening of glucose, lactate, and cholesterol levels. A biofunctionalization method is designed, which provides selectivity towards specific metabolites as well as minimization of any background interference. In addition, a simple method is developed to facilitate multi-analyte sensing and avoid electrical crosstalk between the different transistors by electrically isolating the individual devices. The resulting biosensing platform, verified using human samples, offers the possibility to be used in easy-to-obtain biofluids with low abundance metabolites, such as saliva. Based on our proposed method, other types of enzymatic biosensors can be integrated into the array to achieve multiplexed, noninvasive, personalized point-of-care diagnostics.

From mercury to nanosensors: Past, present and the future perspective of electrochemistry in pharmaceutical and biomedical analysis

Polarography was the first developed automated method of voltage-controlled electrolysis with dropping mercury electrode (DME). Then, hanging mercury drop and static mercury drop electrodes were added as an alternative indicator electrode. In this way, polarography turned formally into voltammetry with mercury electrodes in the electroreduction way. Solid electrodes such as noble metal and carbon based electrodes can be used for the investigation of the compounds for both oxidation and reduction directions, which is called voltammetry. The voltammetric and polarographic techniques are more sensitive, reproducible, and easily used electroanalytical methods that can be alternative to more frequently used separation and spectrometric methods. Furthermore, in some cases there is a relationship between voltammetry and pharmaceutical samples, and the knowledge of the mechanism of their electrode reactions can give a useful clue in elucidation of the mechanism of their interaction with living cells. The voltammetric and polarographic analysis of drugs in pharmaceutical preparations are by far the most common use of electrochemistry for analytical pharmaceutical problems. Recent trends and challenges in the electrochemical methods for the detection of DNA hybridization and pathogens are available. Low cost, small sample requirement and possibility of miniaturization justifies their increasing development.

Biosensors in Health Care: The Milestones Achieved in Their Development towards Lab-on-Chip-Analysis

Immense potentiality of biosensors in medical diagnostics has driven scientists in evolution of biosensor technologies and innovating newer tools in time. The cornerstone of the popularity of biosensors in sensing wide range of biomolecules in medical diagnostics is due to their simplicity in operation, higher sensitivity, ability to perform multiplex analysis, and capability to be integrated with different function by the same chip. There remains a huge challenge to meet the demands of performance and yield to its simplicity and affordability. Ultimate goal stands for providing point-of-care testing facility to the remote areas worldwide, particularly the developing countries. It entails continuous development in technology towards multiplexing ability, fabrication, and miniaturization of biosensor devices so that they can provide lab-on-chip-analysis systems to the community.

PEP-on-DEP: A competitive peptide-based disposable electrochemical aptasensor for renin diagnostics

Antibody-based immunosensors are relatively less accessible to a wide variety of unreachable targets, such as low-molecular-weight biomarkers that represent a rich untapped source of disease-specific diagnostic information. Here, we present a peptide aptamer-based electrochemical sensor technology called 'PEP-on-DEP' to detect less accessible target molecules, such as renin, and to improve the quality of life. Peptide-based aptamers represent a relatively smart class of affinity binders and show great promise in biosensor development. Renin is involved in the regulation of arterial blood pressure and is an emerging biomarker protein for predicting cardiovascular risk and prognosis. To our knowledge, no studies have described aptamer molecules that can be used as new potent probes for renin. Here, we describe a portable electrochemical biosensor platform based on the newly identified peptide aptamer molecules for renin. We constructed a randomized octapeptide library pool with diversified sequences and selected renin specific peptide aptamers using cDNA display technology. We identified a few peptide aptamer sequences with a KD in the µM binding affinity range for renin. Next, we grafted the selected peptide aptamers onto gold nanoparticles and detected renin in a one-step competitive assay using our originally developed DEP (Disposable Electrochemical Printed) chip and a USB powered portable potentiostat system. We successfully detected renin in as little as 300ngmL(-1) using the PEP-on-DEP method. Thus, the generation and characterization of novel probes for unreachable target molecules by merging a newly identified peptide aptamer with electrochemical transduction allowed for the development of a more practical biosensor that, in principle, can be adapted to develop a portable, low-cost and mass-producible biosensor for point-of-care applications.

A novel approach for simultaneous sensing of urea and glucose by SPR based optical fiber multianalyte sensor

A novel design is presented for the sensing of urea and glucose simultaneously by surface plasmon resonance (SPR) based optical fiber multianalyte sensor using gel entrapment technique in a fluid for biomedical applications. Sensing surfaces are prepared by coating of different metals and high index dielectric over-layers (silver and silicon in channel 1 and copper and tin-oxide in channel 2) over two small and well separated unclad portions of an optical fiber. Enzymes urease and glucose oxidase are immobilized over respective surfaces using gel entrapment method for the sensing of urea and glucose, respectively. Wavelength interrogation technique is used for the calibration of the sensor. The concentration of urea and glucose are selected in the range 0-180 mM and 0-260 mg dl(-1), respectively, to mimic the physiological range of urea and glucose in human blood. Mixed solutions of urea and glucose with varying concentrations are prepared in buffer solution for the calibration of the sensor. The resonance wavelengths are determined corresponding to two channels responding independently to the interaction of analytes in solution on sensing channels. It is observed that the resonance wavelengths in the case of both the sensing channels decrease with the increasing concentrations of urea and glucose in solutions. The sensor gives 8.51 nm and 13.04 nm total shifts in resonance wavelength for 0 mM to 180 mM urea and 0 mg dl(-1) to 260 mg dl(-1) glucose concentrations in a sample, respectively. The advantages of the present sensor are cascaded design with multiple sensing channels on a single platform, inexpensive, simultaneous sensing of two parameters, possibility of online monitoring and remote sensing.

A multiple amplification strategy for nucleic acid detection based on host-guest interaction between the β-cyclodextrin polymer and pyrene

A multiple amplification strategy has been developed for nucleic acid detection based on host-guest interaction between the β-cyclodextrin polymer (β-CDP) and pyrene. Briefly, the detection system consists of three parts: the polymerase and nicking enzyme-assisted isothermal strand displacement amplification (SDA) activated by a target DNA or microRNA; the exonuclease III-aided cyclic enzymatic amplification (CEA); and the fluorescence enhancement effect based on host-guest interaction between β-CDP and pyrene. This strategy showed a good positive linear correlation with target DNA concentrations in the range from 75 fM to 1 pM with a detection limit of 41 fM. Significantly, our amplification platform was further validated and evaluated successfully by assaying miRNA-21 in human serum. The proposed assay has great potential as a nucleic acid quantification method for use in biomedical research, clinical analysis and disease diagnostics.

Biosensor technology for pesticides--a review

Pesticides, due to their lucrative outcomes, are majorly implicated in agricultural fields for crop production enhancement. Due to their pest removal properties, pesticides of various classes have been designed to persist in the environment over a longer duration after their application to achieve maximum effectiveness. Apart from their recalcitrant structure and agricultural benefits, pesticides also impose acute toxicological effects onto the other various life forms. Their accumulation in the living system may prove to be detrimental if established in higher concentrations. Thus, their prompt and accurate analysis is a crucial matter of concern. Conventional techniques like chromatographic techniques (HPLC, GC, etc.) used for pesticides detection are associated with various limitations like stumpy sensitivity and efficiency, time consumption, laboriousity, requirement of expensive equipments and highly trained technicians, and many more. So there is a need to recruit the methods which can detect these neurotoxic compounds sensitively, selectively, rapidly, and easily in the field. Present work is a brief review of the pesticide effects, their current usage scenario, permissible limits in various food stuffs and 21st century advancements of biosensor technology for pesticide detection. Due to their exceptional performance capabilities, easiness in operation and on-site working, numerous biosensors have been developed for bio-monitoring of various environmental samples for pesticide evaluation immensely throughout the globe. Till date, based on sensing element (enzyme based, antibody based, etc.) and type of detection method used (Electrochemical, optical, and piezoelectric, etc.), a number of biosensors have been developed for pesticide detection. In present communication, authors have summarized 21st century's approaches of biosensor technology for pesticide detection such as enzyme-based biosensors, immunosensors, aptamers, molecularly imprinted polymers, and biochips technology. Also, the major technological advancements of nanotechnology in the field of biosensor technology are discussed. Various biosensors mentioned in manuscript are found to exhibit storage stability of biocomponent ranging from 30-60 days, detection limit of 10(-6) - 10(-16) M, response time of 1-20 min and applications of developed biosensors in environmental samples (water, food, vegetables, milk, and juice samples, etc.) are also discussed. Researchers all over the globe are working towards the development of different biosensing techniques based on contrast approaches for the detection of pesticides in various environmental samples.

Developing trends in aptamer-based biosensor devices and their applications

Aptamers are, in general, easier to produce, easier to store and are able to bind to a wider variety of targets than antibodies. For these reasons, aptamers are gaining increasing popularity in environmental monitoring as well as disease detection and disease management applications. This review article examines the research and design of RNA and DNA aptamer based biosensor systems and applications as well as their potential for integration in effective biosensor devices. As single stranded DNA or RNA molecules that can bind to specific targets, aptamers are well suited for biomolecular recognition and sensing applications. Beyond being able to be designed for a near endless number of specific targets, aptamers can also be made which change their conformation in a predictable and consistent way upon binding. This can lead to many unique and effective detection methods using a variety of optical and electrochemical means.

Point-of-care platforms

Point-of-care applications are gaining increasing interest in clinical diagnostics and emergency applications. Biosensors are used to monitor the biomolecular interaction process between a disease biomarker and a recognition element such as a reagent. Essential are the quality and selectivity of the recognition elements and assay types used to improve sensitivity and to avoid nonspecific interactions. In addition, quality measures are influenced by the detection principle and the evaluation strategies. For these reasons, this review provides a survey and validation of recognition elements, assays, and various types of detection methods for point-of-care testing (POCT) platforms. Common applications of clinical parameters are discussed and considered. In this ever-changing field, a snapshot of current applications is needed. We provide such a snapshot by way of a table including literature citations and also discuss these applications in more detail throughout.

Fluorescent biosensors: design and application to motor proteins

Reagentless biosensors are single molecular species that report the concentration of a specific target analyte, while having minimal impact on the system being studied. This chapter reviews such biosensors with emphasis on the ones that use fluorescence as readout and can be used for real-time assays of concentration changes with reasonably high time resolution and sensitivity. Reagentless biosensors can be designed with different types of recognition elements, particularly specific binding proteins and nucleic acids, including aptamers. Different ways are described in which a fluorescence signal can be used to report the target concentration. These include the use of single, environmentally sensitive fluorophores; FRET pairs, often used in genetically encoded biosensors; and pairs of identical fluorophores that undergo reversible stacking interactions to change fluorescence intensity. The applications of these biosensors in different types of real-time assays with motor proteins are described together with some specific examples. These encompass regulation and mechanism of motor proteins, using both steady-state assays and single-turnover measurements.

Construction of a bifunctional enzyme fusion for the combined determination of biogenic amines in foods

Biogenic amines (BAs) are a group of low-molecular-mass organic bases derived from free amino acids. Due to the undesirable effects of BAs on human health, amine oxidase-based detection methods for BAs in foods have been developed. Here, we developed a bifunctional enzyme fusion (MAPO) using a Cu(2+)-containing monoamine oxidase (AMAO2) and a flavin adenine dinucleotide-containing putrescine oxidase (APUO) from Arthrobacter aurescens. It was necessary to activate MAPO with supplementary Cu(2+) ions, leading to a 6- to 12-fold improvement in catalytic efficiency (kcat/KM) for monoamines. The optimal temperatures of Cu(2+)-activated MAPO (cMAPO) for both tyramine and putrescine were 50 °C, and the optimal pH values for tyramine and putrescine were pH 7.0 and pH 8.0, respectively, consistent with those of AMAO2 and APUO, respectively. The cMAPO showed relative specific activities of 100, 99, 32, and 32 for 2-phenylethylamine, tyramine, histamine, and putrescine, respectively. The tyramine-equivalent BA contents of fermented soybean pastes by cMAPO were more than 90% of the total BA determined by HPLC. In conclusion, cMAPO is fully bifunctional toward biogenic monoamines and putrescine, allowing the combined determination of multiple BAs in foods. This colorimetric determination method could be useful for point-of-care testing to screen safety-guaranteed products prior to instrumental analyses.

Solid-support immobilization of a "swing" fusion protein for enhanced glucose oxidase catalytic activity

The strategic surface immobilization of a protein can add new functionality to a solid substrate; however, protein activity, e.g., enzymatic activity, can be drastically decreased on immobilization onto a solid surface. The concept of a designed and optimized "molecular interface" is herein introduced in order to address this problem. In this study, molecular interface was designed and constructed with the aim of attaining high enzymatic activity of a solid-surface-immobilized a using the hydrophobin HFBI protein in conjunction with a fusion protein of HFBI attached to glucose oxidase (GOx). The ability of HFBI to form a self-organized membrane on a solid surface in addition to its adhesion properties makes it an ideal candidate for immobilization. The developed fusion protein was also able to form an organized membrane, and its structure and immobilized state on a solid surface were investigated using QCM-D measurements. This method of immobilization showed retention of high enzymatic activity and the ability to control the density of the immobilized enzyme. In this study, we demonstrated the importance of the design and construction of molecular interface for numerous purposes. This method of protein immobilization could be utilized for preparation of high throughput products requiring structurally ordered molecular interfaces, in addition to many other applications.

CD4 counting technologies for HIV therapy monitoring in resource-poor settings--state-of-the-art and emerging microtechnologies

Modern advancements in pharmaceuticals have provided individuals who have been infected with the human immunodeficiency virus (HIV) with the possibility of significantly extending their survival rates. When administered sufficiently soon after infection, antiretroviral therapy (ART) allows medical practitioners to control onset of the symptoms of the associated acquired immune deficiency syndrome (AIDS). Active monitoring of the immune system in both HIV patients and individuals who are regarded as "at-risk" is critical in the decision making process for when to start a patient on ART. A reliable and common method for such monitoring is to observe any decline in the number of CD4 expressing T-helper cells in the blood of a patient. However, the technology, expertise, infrastructure and costs to carry out such a diagnostic cannot be handled by medical services in resource-poor regions where HIV is endemic. Addressing this shortfall, commercialized point-of-care (POC) CD4 cell count systems are now available in such regions. A number of newer devices will also soon be on the market, some the result of recent maturing of charity-funded initiatives. Many of the current and imminent devices are enabled by microfluidic solutions, and this review will critically survey and analyze these POC technologies for CD4 counting, both on-market and near-to-market deployment. Additionally, promising technologies under development that may usher in a new generation of devices will be presented.

Imidazolium-based ionic liquid surfaces for biosensing

Ionic liquid self-assembled monolayers (SAM) were designed and applied for binding streptavidin, promoting affinity biosensing and enzyme activity on gold surfaces of sensors. The synthesis of 1-((+)-biotin)pentanamido)propyl)-3-(12-mercaptododecyl)-imidazolium bromide, a biotinylated ionic liquid (IL-biotin), which self-assembles on gold film, afforded streptavidin sensing with surface plasmon resonance (SPR). The IL-biotin-SAM efficiently formed a full streptavidin monolayer. The synthesis of 1-(carboxymethyl)-3-(mercaptododecyl)-imidazoliumbromide, a carboxylated IL (IL-COOH), was used to immobilize anti-IgG to create an affinity biosensor. The IL-COOH demonstrated efficient detection of IgG in the nanomolar concentration range, similar to the alkylthiols SAM and PEG. In addition, the IL-COOH demonstrated low fouling in crude serum, to a level equivalent to PEG. The IL-COOH was further modified with N,N'-bis (carboxymethyl)-l-lysine hydrate to bind copper ions and then, chelate histidine-tagged biomolecules. Human dihydrofolate reductase (hDHFR) was chelated to the modified IL-COOH. By monitoring enzyme activity in situ on the SPR sensor, it was revealed that the IL-COOH SAM improved the activity of hDHFR by 24% in comparison to classical SAM. Thereby, IL-SAM has been synthesized and successfully applied to three important biosensing schemes, demonstrating the advantages of this new class of monolayers.

Tethered proteoliposomes containing human ABC transporter MRP3: new perspectives for biosensor application based on transmembrane proteins

While transmembrane proteins and transporters comprise one of the largest protein families, their use in biosensors like biochips or lab-on-a-chip devices has so far been limited by their demanding requirements of a stable and compartmentalized lipid environment. A possible remedy lies in the tethering of proteoliposomes containing the reconstituted transmembrane protein to the biosensoric surface. As a proof of concept, we reconstituted the human ABC transporter MRP3 into biotinylated proteoliposomes and tethered those to a gold surface coated with streptavidin on a biotinylated self-assembled thiol monolayer. The tethering process was investigated by quartz crystal microbalance with dissipation monitoring. The final assembly of tethered proteoliposomes exhibited biological activity in terms of drug-stimulated ATP hydrolysis and substrate translocation. The presented facile immobilization approach can be easily extended to other transmembrane proteins as it does not require any modification of the protein and will open up transmembrane proteins for future application in biosensors.