Biosensors that detect and convert biological reactions to a measurable signal have gained much attention in recent years. Between 1950 and 2017, more than 150,000 papers have been published addressing the applications of biosensors in different industries, but to the best of our knowledge and through careful screening, critical reviews that describe hydrogel based biosensors for biomedical applications are rare. This review discusses the biomedical application of hydrogel based biosensors, based on a search performed through Web of Science Core, PubMed (NLM), and Science Direct online databases for the years 2000⁻2017. In this review, we consider bioreceptors to be immobilized on hydrogel based biosensors, their advantages and disadvantages, and immobilization techniques. We identify the hydrogels that are most favored for this type of biosensor, as well as the predominant transduction strategies. We explain biomedical applications of hydrogel based biosensors including cell metabolite and pathogen detection, tissue engineering, wound healing, and cancer monitoring, and strategies for small biomolecules such as glucose, lactate, urea, and cholesterol detection are identified.

https://www.mdpi.com/2073-4360/9/8/364/pdf

Hydrogel Based Sensors for Biomedical Applications: An Updated Review.

Understanding the molecular mechanisms for the enhanced phytoremediation of heavy metals through plant growth promoting rhizobacteria: A review.

Plant growth promoting rhizobacteria (PGPRs) are very effective in immobilization of heavy metals and reducing their translocation in plants via precipitation, complex formation and adsorption. The present study was therefore designed to understand the role of Pseudomonas aeruginosa and Burkholderia gladioli in mitigation of Cd stress (0.4 mM) in 10-days old L. esculentum seedlings. The present work investigated growth characteristics, photosynthetic pigments, metal tolerance index, metal uptake and the contents of metal chelating compounds (protein bound and non-protein bound thiols, total thiols) in microbes inoculated Cd treated L. esculentum seedlings. The gene expression profiling of different metal transporters was conducted in order to investigate the quantitative analysis. Our results revealed Cd generated toxicity in seedlings in terms of reduced growth (root length, shoot length and fresh weight) and photosynthetic pigments (chlorophyll, carotenoid and xanthophyll) which enhanced upon inoculations of P. aeruginosa and B. gladioli. Further, the metal uptake along with levels of protein and non-protein bound thiols was also enhanced in Cd-treated seedlings. Gene expression studies suggested enhanced expression in the metal transporter genes which were further declined in the microbe supplemented seedlings. Therefore, micro-organisms possess growth promoting traits that enable them to reduce metal toxicity in plants.

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Metal resistant PGPR lowered Cd uptake and expression of metal transporter genes with improved growth and photosynthetic pigments in Lycopersicon esculentum under metal toxicity.

Biosensor development includes the deposition of (nano)materials onto a conductive electrode surface, which is a crucial step for obtaining improved performance from the constructed biosensors. Various methods have been used to create a successful matrix of (nano)materials that ensures proper contact between the material and electrode surface. The purpose of (nano)material deposition is to provide a high surface area to improve the electroanalytical performance of biosensors by supporting the stable immobilization of enzymes in a more significant quantity as well as enhancing the catalytic or bioaffinity features. For decades, researchers have been using increasingly advanced methods not only for improving sensing performance, but also for improving stability, reproducibility, and mass production. In this review, we summarized the methods used for (nano)material deposition onto an electrode surface for efficient biosensor fabrication. An enhanced and optimized (nano)material deposition method is crucial for the mechanical stability and fabrication reproducibility of electrodes when designing a suitable biosensing device. In addition, we discussed the problems faced during biosensor application as well as the present challenges and prospects for superior deposition methods.

Deposition of nanomaterials: A crucial step in biosensor fabrication

In the present study, we have developed a smartphone based handheld optical biosensor for determination of urea in saliva. A simple strategy was adopted by immobilization of urease enzyme along with a pH indicator on a filter paper based strip. The strip changed color upon the reaction with urea present in saliva and the color change can be estimated using our smartphone based application based on RGB profiling. Calibration of the biosensor was carried out using spiked saliva samples and an exponentially decreasing calibration curve has been obtained for green pixel intensity in the broad range (10–1000 mgdL−1) with a linear detection range of 10–260 mgdL−1 and a response time of 20 s. The sensitivity reported for the biosensor in the clinically significant range was −0.005 average pixels sec−1/mgdL−1 with a LOD of 10.4 mgdL−1. Studies carried out on spiked saliva samples showed a good correlation between salivary urea estimated using our biosensor against phenol-hypochlorite based spectroscopic procedure. Development of a smartphone based biosensor for urea estimation eliminates the need for procuring a dedicated instrument as well as trained technician for daily monitoring and saves time as compared to traditional laboratory methods of analysis.

Smartphone based optical biosensor for the detection of urea in saliva

Remediation of arsenic in mung bean (Vigna radiata) with growth enhancement by unique arsenic-resistant bacterium Acinetobacter lwoffii.

Conducting polymer hydrogels (CPH) are a unique class of materials, which have both the advantageous features of hydrogels and organic conductors. Herein, we report a new urea biosensor in which the working electrode is modified by a CPH, specifically a composite made of conducting electro-polymerized aniline in the non-conducting hydrogel of polyacrylamide and polyvinyl alcohol for immobilization of the enzyme urease. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies depict the porous structure of the hydrogel, which favours the high density immobilization of enzymes. Fourier transform infrared spectroscopy (FTIR) studies confirmed the grafting of polyaniline within the matrix. Electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV) and cyclic voltammetry (CV) predict high enzyme loading in the hydrogel. Bode plot analysis with the [FeCN6]3−/4− redox couple shows the partial capacitive behaviour of the bioelectrode in the frequency range of 102 to 103 Hz. Factors, such as scan rate and pH of the electrolyte, were optimized to give the best results. DPV could detect 1.5–1000 μM urea with the lowest detection limit (LOD) of 60 nM and sensitivity of 878 μA mM−1 cm−2. The relatively low Michaelis–Menten constant (Km) value of 113 μM indicates the high affinity of the enzyme for urea. EIS depicts an increase in charge transfer resistance (RCT) with an increase in urea concentration (LOD = 14 μM). The proposed sensor exhibits a very good performance with respect to detection limit, sensitivity, Km value, preparation method, cost, sample preparation, stability, selectivity and reproducibility. This sensor was successfully applied for the detection of urea in milk, puffed rice, soil and human blood. Statistical analysis of the results shows good agreement with commercial spectrophotometric analysis.

Enzymatic electrochemical biosensor for urea with a polyaniline grafted conducting hydrogel composite modified electrode

Arsenic, a common contaminant of groundwater, affects human health adversely. According to the World Health Organization (WHO), the maximum recommended contamination level of arsenic in drinking water is 10 μg/L. The purpose of this research was to develop user-friendly kits for detection of arsenic to measure at least up to 10 μg/L in drinking water, so that a preventive measure could be taken. Two different kits for detection of total arsenic in water are reported here. First, the arsenic in drinking water was converted to arsine gas by a strong reducing agent. The arsine produced was then detected by paper strips via generation of color due to reaction with either mercuric bromide (KIT-1) or silver nitrate (KIT-2). These were previously immobilized on the detector strip. The first one gave a yellow color and the second one grey. Both of these kits could detect arsenic contamination within a range of 10 μg/L-250 μg/L. The detection time for both the kits was only 7 min. The kits exhibited excellent performance compared to other kits available in the market with respect to detection time, ease of operation, cost and could be easily handled by a layman. The field trials with these kits gave very satisfactory results. A study on interference revealed that these kits could be used in the presence of 24 common ions present in the arsenic contaminated water. Though the kits were meant for qualitative assay, the results with unknown concentrations of real samples, when compared with atomic absorption spectrophotometer (AAS) were in good agreement as revealed by the t-test.

Low-cost field test kits for arsenic detection in water

The present work reports on a polymer hydrogel-based cost effective colorimetric dipstick sensor for arsenic(V). The method was based on the formation of a blue colored antimonyl–arseno–molybdate complex in the presence of ammonium molybdate, potassium antimonyl tartrate and ascorbic acid. All these reagents were encapsulated in a polymer hydrogel made of polyvinyl alcohol, acrylamide and glutaraldehyde. Plastic detector strips were dip coated with this hydrogel. Scanning electron microscopy (SEM) depicted the highly porous structure of the polymer hydrogel permitting adsorption of As(V) confirmed by energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) analysis. The advantages were low sample volume (10 ml), low detection limit (10 μg L−1 by the naked eye), good stability (4 months), low cost (0.03$ per test for visual detection), and high reproducibility. The sensor strips displayed selectivity in the presence of <100 μg L−1 of phosphate (arsenate structural analog) and 3 mg L−1 of iron. Changes in the RGB (red, green, and blue) values of color spots on the As(V) colorimetric dipstick sensor with various concentrations of As(V) created a pattern. The dominance of B (blue) values was revealed in low concentration range of As(V) while the R (red) values dominated in higher concentrations. The same strip could be used for detection of total arsenic in case the sample contained As(III). Phosphate interference could also be detected by a simple modification of the procedure. When applied to real groundwater arsenic contaminated samples, the results were in good agreement with the atomic absorption spectrophotometry (AAS) results as visualized by statistical analysis.

Joyati Das

Papers

Remediation of arsenic in mung bean (Vigna radiata) with growth enhancement by unique arsenic-resistant bacterium Acinetobacter lwoffii.

Enzymatic electrochemical biosensor for urea with a polyaniline grafted conducting hydrogel composite modified electrode

Low-cost field test kits for arsenic detection in water

A new dipstick colorimetric sensor for detection of arsenate in drinking water

Recent advances in sensors for detecting food pathogens, contaminants, and toxins: a review