Quantitative impedimetric immunosensor for free and total prostate specific antigen based on a lateral flow assay format

Integrated electrochemical lateral flow immunoassays (eLFIAs): recent advances

Schematic of integrated electrochemical lateral flow immunoassays.

Integrating high-performing electrochemical transducers in lateral flow assay

Lateral flow assays (LFAs) are the best-performing and best-known point-of-care tests worldwide. Over the last decade, they have experienced an increasing interest by researchers towards improving their analytical performance while maintaining their robust assay platform. Commercially, visual and optical detection strategies dominate, but it is especially the research on integrating electrochemical (EC) approaches that may have a chance to significantly improve an LFA's performance that is needed in order to detect analytes reliably at lower concentrations than currently possible. In fact, EC-LFAs offer advantages in terms of quantitative determination, low-cost, high sensitivity, and even simple, label-free strategies. Here, the various configurations of EC-LFAs published are summarized and critically evaluated. In short, most of them rely on applying conventional transducers, e.g., screen-printed electrode, to ensure reliability of the assay, and additional advances are afforded by the beneficial features of nanomaterials. It is predicted that these will be further implemented in EC-LFAs as high-performance transducers. Considering the low cost of point-of-care devices, it becomes even more important to also identify strategies that efficiently integrate nanomaterials into EC-LFAs in a high-throughput manner while maintaining their favorable analytical performance. Graphical abstract.

Integrating high-performing electrochemical transducers in lateral flow assay

Lateral flow assays (LFAs) are the best-performing and best-known point-of-care tests worldwide. Over the last decade, they have experienced an increasing interest by researchers towards improving their analytical performance while maintaining their robust assay platform. Commercially, visual and optical detection strategies dominate, but it is especially the research on integrating electrochemical (EC) approaches that may have a chance to significantly improve an LFA’s performance that is needed in order to detect analytes reliably at lower concentrations than currently possible. In fact, EC-LFAs offer advantages in terms of quantitative determination, low-cost, high sensitivity, and even simple, label-free strategies. Here, the various configurations of EC-LFAs published are summarized and critically evaluated. In short, most of them rely on applying conventional transducers, e.g., screen-printed electrode, to ensure reliability of the assay, and additional advances are afforded by the beneficial features of nanomaterials. It is predicted that these will be further implemented in EC-LFAs as high-performance transducers. Considering the low cost of point-of-care devices, it becomes even more important to also identify strategies that efficiently integrate nanomaterials into EC-LFAs in a high-throughput manner while maintaining their favorable analytical performance.

A comparative Study of Aptasensor Vs Immunosensor for Label-Free PSA Cancer Detection on GQDs-AuNRs Modified Screen-Printed Electrodes

Label-free and sensitive detection of PSA (Prostate Specific Antigen) is still a big challenge in the arena of prostate cancer diagnosis in males. We present a comparative study for label-free PSA aptasensor and PSA immunosensor for the PSA-specific monoclonal antibody, based on graphene quantum dots-gold nanorods (GQDs-AuNRs) modified screen-printed electrodes. GQDs-AuNRs composite has been synthesized and used as an electro-active material, which shows fast electron transfer and catalytic property. Aptamer or anti-PSA has immobilized onto the surface of modified screen printed electrodes. Three techniques are used simultaneously, viz. cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedence spectroscopy (EIS) to investigate the analytical performance of both PSA aptasensor and PSA immunosensor with its corresponding PSA antigen. Under optimum conditions, both sensors show comparable results with an almost same limit of detection (LOD) of 0.14 ng mL-1. The results developed with aptasensor and anti-PSA is also checked through the detection of PSA in real samples with acceptable results. Our study suggests some advantages of aptasensor in terms of better stability, simplicity and cost effectiveness. Further our present work shows enormous potential of our developed sensors for real application using voltammetric and EIS techniques simultaneous to get reliable detection of the disease.

Development of a SERS-Based Rapid Vertical Flow Assay for Point-of-Care Diagnostics

Point-of-care (POC) diagnostic testing platforms are a growing sector of the healthcare industry as they offer the advantages of rapid provision of results, ease of use, reduced cost, and the ability to link patients to care. While many POC tests are based on chromatographic flow assay technology, this technology suffers from a lack of sensitivity along with limited capacity for multiplexing and quantitative analysis. Several recent reports have begun to investigate the feasibility of coupling chromatographic flow platforms to more advanced read-out technologies which in turn enable on-site acquisition, storage, and transmission of important healthcare metrics. One such technology being explored is surface-enhanced Raman spectroscopy or SERS. In this work, SERS is coupled for the first time to a rapid vertical flow (RVF) immunotechnology for detection of anti-HCV antibodies in an effort to extend the capabilities of this commercially available diagnostic platform. High-quality and reproducible SERS spectra were obtained using reporter-modified gold nanoparticles (AuNPs). Serial dilution studies indicate that the coupling of SERS with RVF technology shows enormous potential for next-generation POC diagnostics.

Sensitivity improvement of a sandwich-type ELISA immunosensor for the detection of different prostate-specific antigen isoforms in human serum using electrochemical impedance spectroscopy and an ordered and hierarchically organized interfacial supramolecular architecture

A gold millielectrode (GME) functionalized with a mixed (16-MHA + EG3SH) self-assembled monolayer (SAM) was used to fabricate an indirect enzyme-linked immunosorbent assay (ELISA) immunosensor for the sensitive detection of prostate-specific antigen (PSA), a prostate cancer (PCa) biomarker, in human serum samples. To address and minimize the issue of non-specific protein adsorption, an organic matrix (amine-PEG3-biotin/avidin) was assembled on the previously functionalized electrode surface to build up an ordered and hierarchically organized interfacial supramolecular architecture: Au/16-MHA/EG3SH/amine-PEG3-biotin/avidin. The electrode was then exposed to serum samples at different concentrations of a sandwich-type immunocomplex molecule ((Btn)Ab-AgPSA-(HRP)Ab), and its interfacial properties were characterized using electrochemical impedance spectroscopy (EIS). Calibration curves for polarization resistance (RP) and capacitance (1/C) vs. total and free PSA concentrations were obtained and their analytical quality parameters were determined. This approach was compared with results obtained from a commercially available ELISA immunosensor. The results obtained in this work showed that the proposed immunosensor can be successfully applied to analyze serum samples of patients representative of the Mexican population.

Surface stress-based biosensors

Surface stress-based biosensors, as one kind of label-free biosensors, have attracted lots of attention in the process of information gathering and measurement for the biological, chemical and medical application with the development of technology and society. This kind of biosensors offers many advantages such as short response time (less than milliseconds) and a typical sensitivity at nanogram, picoliter, femtojoule and attomolar level. Furthermore, it simplifies sample preparation and testing procedures. In this work, progress made towards the use of surface stress-based biosensors for achieving better performance is critically reviewed, including our recent achievement, the optimally circular membrane-based biosensors and biosensor array. The further scientific and technological challenges in this field are also summarized. Critical remark and future steps towards the ultimate surface stress-based biosensors are addressed.

Targeting free prostate-specific antigen for in vivo imaging of prostate cancer using a monoclonal antibody specific for unique epitopes accessible on free prostate-specific antigen alone

This study investigated the feasibility of targeting the free, unbound forms of prostate-specific antigen (fPSA) for in vivo imaging of prostate adenocarcinomas (PCa), as PSA is produced and secreted at abundance during every clinical stage and grade of PCa, including castration-resistant disease. We injected (125)I-labeled monoclonal antibody PSA30 (specific for an epitope uniquely accessible on fPSA alone) intravenously in male nude mice carrying subcutaneous xenografts of LNCaP tumors (n=36). Mice were sacrificed over a time course from 4 hours to 13 days after injecting (125)I-labeled PSA30. Tissue uptake of (125)I-PSA30 at 48 and 168 hours after intravenous injection was compared with two clinically used positron emission tomography radiopharmaceuticals, (18)F-fluoro-deoxy-glucose ((18)F-FDG) or (18)F-choline, in cryosections using Digital AutoRadiography (DAR) and also compared with immunohistochemical staining of PSA and histopathology. On DAR, the areas with high (125)I-PSA30 uptake corresponded mainly to morphologically intact and PSA-producing LNCaP cells, but did not associate with the areas of high uptake of either (18)F-FDG or (18)F-choline. Biodistribution of (125)I-PSA30 measured in dissected organs ex vivo during 4 to 312 hours after intravenous injection demonstrated maximum selective tumor uptake 24-48 hours after antibody injection. Our data showed selective uptake in vivo of a monoclonal antibody highly specific for fPSA in LNCaP cells. Hence, in vivo imaging of fPSA may be feasible with putative usefulness in disseminated PCa.

An aptamer based resonance light scattering assay of prostate specific antigen

Prostate specific antigen (PSA) is a valuable tumor marker for prostate cancer screening. In this work, a novel and sensitive resonance light scattering (RLS) spectral assay of PSA was proposed based on PSA aptamer modified gold nanoparticles (AuNPs). The sulfhydryl modified single-strand aptamer could interact with AuNPs, which made the AuNPs stable in high concentration of salt. In pH 7.0 BR buffer solution, the highly selective combination of PSA and AuNPs-labeling aptamer resulted in the aggregation of AuNPs which showed high RLS intensity. Under the optimal conditions, the magnitude of enhanced RLS intensity (ΔI(RLS)) was proportional to the concentration of PSA in the range from 0.13 to 110 ng/mL, with a detection limit (LOD, 3σ) of 0.032 ng/mL. This developed RLS assay as well as a commercially available enzyme-linked immunosorbent assay (ELISA) kit was successfully applied to the detection of PSA in 15 serum samples, and an excellent correlation of the levels of PSA measured was obtained. This is the first report of the aptamer based RLS assay for PSA and it is also a significant application of instrumental analysis technique.

Microfluidic systems for biosensing

In the past two decades, Micro Fluidic Systems (MFS) have emerged as a powerful tool for biosensing, particularly in enriching and purifying molecules and cells in biological samples. Compared with conventional sensing techniques, distinctive advantages of using MFS for biomedicine include ultra-high sensitivity, higher throughput, in-situ monitoring and lower cost. This review aims to summarize the recent advancements in two major types of micro fluidic systems, continuous and discrete MFS, as well as their biomedical applications. The state-of-the-art of active and passive mechanisms of fluid manipulation for mixing, separation, purification and concentration will also be elaborated. Future trends of using MFS in detection at molecular or cellular level, especially in stem cell therapy, tissue engineering and regenerative medicine, are also prospected.

Monitoring the activation of neuronal nitric oxide synthase in brain tissue and cells with a potentiometric immunosensor

An all solid state potentiometric immunosensor (ASPI) has been developed to study the activation process of neuronal nitric oxide synthase (nNOS), the enzyme involved in the synthesis of nitric oxide generated under physiological conditions. At first, an all solid state H(+)-selective ISE was fabricated with the carboxylated poly(vinyl chloride) (PVC-COOH) film containing H(+) ionophore, antibody was then immobilized on the polymer layer. The immunocomplex formation was detected by monitoring pH change due to interaction between urease labeled secondary antibody and antigen. Experimental parameters such as the amount of phosphorylated nNOS immobilized on the electrode surface and pH responses due to the antibody-antigen reaction were studied in detail. The calibration plot of the potentiometric potential vs. phosphorylated nNOS concentration exhibited a linear relationship in the range of 3.4-340.0 microg/ml. The calibration sensitivity of the phosphorylated nNOS immunosensor was -0.073+/-0.002 mV/microg ml(-1). The detection limit of nNOS was determined to be 0.2 microg/ml based on five-time measurements (95% confidence level, k=3, n=5). The reliability of the immunosensor was examined with rat brain tissues as well as neuronal cells, and the results shown were good, implying a promising approach for a novel electrochemical immunosensor platform with potential applications to clinical diagnosis.

Over-the-Counter Biosensors: Past, Present, and Future

The demand for specific, low cost, rapid, sensitive and easy detection of biomolecules is huge. A well-known example is the glucose meters used by diabetics to monitor their blood glucose levels. Nowadays, a vast majority of the glucose meters are based on electrochemical biosensor technology. The inherent small size and simple construction of the electrochemical transducer and instrument are ideally suited for point-of-care biosensing. Besides glucose, a wide variety of electrochemical biosensors have been developed for the measurements of some other key metabolites, proteins, and nucleic acids. Nevertheless, unlike the glucose meters, limited success has been achieved for the commercialization of the protein and nucleic acid biosensors. In this review article, key technologies on the electrochemical detection of key metabolites, proteins, and DNAs are discussed in detail, with particular emphasis on those that are compatible to home-use setting. Moreover, emerging technologies of lab-on-a-chip microdevices and nanosensors (i.e., silicon and carbon nanotube field-effect sensors) offer opportunities for the construction of new generation biosensors with much better performances. Together with the continuous innovations in the basic components of biosensors (i.e., transducers, biorecognition molecules, immobilization and signal transduction schemes), consumers could soon buy different kinds of biosensing devices in the pharmacy stores.

Labeless immunosensor assay for prostate specific antigen with picogram per milliliter limits of detection based upon an ac impedance protocol

This paper describes the development of labeless immunosensors for the prostate cancer marker prostate specific antigen (PSA). Poly(1,2-diaminobenzene) was electrodeposited onto screen-printed carbon electrodes, and this modified surface was sonochemically ablated to form a microelectrode array. Polyaniline was electropolymerized within these pores to form a microarray of conductive polyaniline protrusions. Two methods were utilized to immobilize antibodies for prostate specific antigen (APSA). The first involved entrapment of APSA during electropolymerization of the polyaniline. The second utilized a polyaniline array as a substrate to immobilize a biotinylated APSA using a classical avidin-biotin affinity approach. Microelectrode arrays were interrogated using ac impedance protocols before and following exposure to PSA solutions. Our preliminary results show that concentration/ac response relationships were recorded over very different ranges; sensors fabricated using an affinity approach exhibited detection limits 1000 times lower than those formulated by the entrapment method. This demonstrates that assembly protocols have major effects on immunosensor performance.

Long-wavelength fluorescence polarization immunoassay: determination of amikacin on solid surface and gliadins in solution

The versatility of the fluorescence polarization immunoassay (FPIA) is increased by using two long-wavelength labels, Nile Blue and a ruthenium(II) chelate. The first label has been used to study the potential of FPIA on a solid surface using dry reagent technology. The aminoglycoside antibiotic amikacin has been used as an analyte model, and the method has been applied to the analysis of serum samples. The second label has been used to show the practical application of FPIA to the determination of macromolecules, using gliadins as an analyte model, which have been determined in gluten-free food. Very low amounts of anti-amikacin antibodies and amikacin-Nile Blue tracer were immobilized onto nitrocellulose membranes, for the development of the amikacin method, and the consumption of reagents is lower than in conventional FPIA. Only the addition of the standard or sample extract at an adequate pH is required at the analysis time. The analyte displaces the tracer from the tracer-antibody immunocomplex, obtaining a decrease in the fluorescence polarization proportional to the analyte concentration. The gliadin-Ru(II) chelate tracer shows a relatively long lifetime, which allows the observation of differences in fluorescence polarization values between the tracer-antibody complex and the tracer alone. The dynamic range of the calibration graphs for both analytes is 0.5-10 microg mL-1 and the detection limits are 0.1 and 0.09 microg mL-1 for amikacin and gliadins, respectively. The study of the precision gave values of relative standard deviations lower than 5 and 1.5% for the amikacin and gliadin methods, respectively. Amikacin was determined in human serum samples using a previous deproteinization step with acetonitrile, obtaining recovery values in the range 83.4-122.8%. The gliadin method was applied to the analysis of gluten-free food samples by using a previous extraction step. The recovery study gave values between 94.3 and 105.0%.

A bio-barcode assay for on-chip attomolar-sensitivity protein detection

Functionalized nanoparticles hold great promise in realizing highly sensitive and selective biodetection. We report a single disposable chip which is capable of carrying out a multi-step process that employs nanoparticles--a bio-barcode assay (BCA) for single protein marker detection. To illustrate the capability of the system, we tested for the presence of prostate specific antigen (PSA) in buffer solution and goat serum. Detection was accomplished at PSA concentrations as low as 500 aM. This corresponds to only 300 copies of protein analytes using 1 microL total sample volume. We established that the on-chip BCA for PSA detection offers four orders of magnitude higher sensitivity compared to commercially available ELISA-based PSA tests.

Disposable noncompetitive immunosensor for free and total prostate-specific antigen based on capacitance measurement

This work reports on the successful integration of a one-step lateral flow immunoassay format and impedance detection of the specific affinity event using an electrochemical transducer coated with a pH-sensitive polymer layer. This approach was particularly applied to the development of a rapid single-use immunosensor for the sensitive detection of free and total prostate-specific antigen (f-PSA, t-PSA) tumor marker. Strips of nitrocellulose membrane were coated with appropriate antibodies to f-PSA and t-PSA and used as solid supports for the performance of noncompetitive immunoassays where PSA was allowed to react with both immobilized anti-PSA antibody and anti-PSA urease enzyme conjugate for less than 1 min. An additional piece within the device consisting of a storage blister filled with a urea solution allowed the rapid washing of unbound species from the membrane strips and simultaneous urea hydrolysis catalyzed by the bound urease conjugate in an automatic fashion. The hydrolysis of urea increased the pH of the reaction media, which in turn induced a breakdown of the polymer layer on the transducer and a consequent measurable change in capacitance of the system. This was easily recorded at a given frequency over a 30-min period. Overall, we describe a one-step immunosensor prototype that exhibits enough sensitivity to detect both forms of PSA at concentration levels down to 3 ng/mL. With the possibility of being portable and considering its ease of use, robustness, and simplicity, this device has great potential as a tool for the screening and early detection of prostate cancer.

One-step immunostrip test for the simultaneous detection of free and total prostate specific antigen in serum

The development of a one-step lateral flow immunoassay on a strip format for the rapid and reliable simultaneous detection of serum levels of free and total prostate specific antigen (f-PSA and t-PSA) and estimation of f-PSA to t-PSA ratio (f/t-PSA) is reported. The f/t-PSA ratio has shown to be more specific for the correct diagnosis of prostate cancer than t-PSA alone, especially in the so-called diagnostic grey zone of 4-10 microg/l t-PSA. The performance of the system described relied on non-competitive immunoassay protocols. Herein, f-PSA and t-PSA were sandwiched between anti-f-PSA and anti-t-PSA monoclonal antibodies immobilised on the strip and a colloidal gold anti-PSA antibody tracer. In the presence of PSA in the sample, the tracer accumulated on the strip results in the appearance of specific pink colour lines. The colour intensity of these lines was found to be directly proportional to the PSA concentration and a semi-quantitative estimation could be carried out visually. Quantitative analysis was also possible by densitometry. Using PSA standards prepared in female serum, the strip could be calibrated up to a concentration of 60 microg/l for both PSA species, with an assay time of less than 20 min. The estimated detection limit was 1 microg/l in all cases. The immunostrip showed good storage stability for at least 2 months and the reproducibility was always between 12% and 17%. Fifty-one male serum samples were analysed with the strip and results compared with values obtained by two different commercial immunoassays taken as reference methods. The study yielded acceptable correlation and agreement. An estimation of the sensitivity and specificity demonstrated the strip validity as a potential front-line device for the early detection of prostate cancer and differentiation of benign prostatic anomalies. Small plastic cartridges incorporating the immunostrip and a small blister containing washing solution that helps remove unbound species from the strip were envisaged in order to avoid false positive readings and decrease background signals, thereby leading to better sensitivity and detection limits.

Impedimetric immunosensors based on the conjugated polymer PPV

In the work reported here, we investigated the interaction between the semiconducting polymer MDMO-PPV and antibodies against the fluorescent dyes fluorescein isothiocyanate (FITC) and Cy5. The antibodies are adsorbed physically onto thin polymer films on gold electrodes, as seen in AFM images of these films. By tuning the antibody concentration, the contact angle of distilled water with the film can be made to vary between 95 degrees and 50 degrees, showing that different surface densities of antibody can be obtained. That these biosensor films specifically bind their antigenic fluorescent molecules from PBS buffer solution is demonstrated by confocal fluorescence microscopy. Specific antigen-antibody recognition is demonstrated by lack of cross-sensitivity between the two antibodies and their antigens. In a biosensor prototype based on differential impedance spectroscopy, these polymer films show a clear response to 1 ppb antigen solution, with a time constant of 2-3 min.