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In this paper we demonstrate an efficient and non-interfering computational method for sub-femtomolar food toxin detection in complex mixture based on nanoporous silicon oxide impedance immunosensor by employing noise spectroscopy analysis at the peak frequency. It has been observed that the peak frequency ( f p ) values obtained from steady state impedance measurements cannot distinguish between solution with only the specific toxin, which is aflatoxin B1 (AfB1) and mixture of AfB1 with other non-specific toxins (NSTs), thus leading to erroneous quantification of AfB1 in complex mixture. On the other hand, the first cut-off frequency ( f c ) ranges obtained from noise spectroscopy analysis can qualitatively differentiate between solution containing only AfB1, AfB1 and NSTs and no AfB1. However f c values being very close for different concentration of AfB1 in pure solution and being overlapping for different mixtures cannot quantify AfB1 either in pure solution or in complex mixture. To address this problem, the proposed computational method first clusters the f p and f c values in 11 categories each using k -means clustering algorithm and then applies a simple combinational digital logic on the clusters of f p s and f c s to obtain the final output, realizable with standard NAND–NOR gates. The output digital word differs only with AfB1 concentration and not with concentration of NSTs and is found to be capable of detecting sub-femtomolar AfB1 range down to 0.1 fg/ml not only in pure solution but also in complex mixture with as high as 1000 ng/ml NSTs. This is the most sensitive and selective report so far on electrochemical food toxin immunosensors. [ABSTRACT FROM AUTHOR]

The use of porous silicon (PS) as an electrochemical biosensor has been reviewed. For electrochemical biosensing, the enhanced surface area is not the only advantage of using PS. In fact, depending on the transduction mechanisms, various issues like modulation of electric field lines through the pores and its interface with the biomolecules and ions of the analytes, nature of PS–metal contacts, orientation of the molecules within the pores and modification of the effective spring constant of the structure leading to a change in the resonant frequency have significant impact on sensitivity, reproducibility and longevity. In this review, we provide a critical assessment of the various factors which are actually responsible for enhanced performance of PS electrochemical biosensors. Formation procedures of PS for electrochemical biosensing have been mentioned. The mechanism of surface modification and derivatization has been briefly discussed. The contact formation techniques have been outlined. Four different categories of electrochemical biosensors using PS have been analysed in details with emphasis on equivalent circuit models and the specific role of PS in the electrical transduction mechanisms. Finally the existing challenges and the future prospects of such devices for commercial use are highlighted. [ABSTRACT FROM AUTHOR]

Abstract: Microimpedance affinity biosensors using interdigitated electrodes are being extensively employed for label free and rapid detection of bacteria molecules. To improve the sensitivity of such sensors, it is required to maximize the antibody immobilization and bacteria captured for an applied input concentration through proper quantification and also optimize the electrode geometry to achieve maximum signal to noise ratio. In this paper, a biomolecule compatible electrical model has been developed which quantifies the number of immobilized antibody and captured bacteria by simple calculations from impedance spectroscopy results which are comparable to the accuracy obtained by conventional time consuming and sometimes expensive biochemical methods and can also estimate the electrode impedance associated with each biomolecule. The model has been applied for the detection of Escherichia coli K12 bacteria by antibody–antigen binding method on gold electrodes with PBS as the background solution. The quantitative estimation of the actual number of antibody immobilized and bacteria captured for input concentration of 103 and 106 CFU/ml on the gold surface in this case has been matched with the conventional optical density methods and radioactive labeling method respectively. A quantitative relationship between the geometrical parameters and the output signal has been deduced after extracting the electrode impedance for each biomolecule and the effects of transducer geometry on the sensitivity of this biosensor have been matched with recently reported experimental results for the same system. Thus the proposed model enables quantitative estimation of the antibody immobilized and bacteria captured and quantitative prediction of the optimum geometry for maximum sensitivity eliminating the need for trial and error experimentation. This will lead to a rapid and cost effective method for performance enhancement of microimpedance biosensors with different topologies. [Copyright &y& Elsevier]

In this paper we report the sensing of aflatoxin B1(AFB1) by field effect capacitive method using electrophoretically deposited reduced graphene oxide (RGO) films for the first time. The RGO film has been characterized using SEM, surface profilometer and Raman spectroscopy. It has been observed that both quantum capacitance of RGO ( C q ) and effective electrical double layer capacitance ( C EDL ) contribute significantly towards the overall sensitivity for molar concentration in the range of 20–50 mM. As C q and C EDL changes in opposite direction after AFB1 capture and the nature of frequency dependence of C q and C EDL are different, the sensitivity shows a minima at a particular frequency. Interestingly, the sensitivity minima is also dependent on AFB1 concentration. Further, the maximum sensitivity obtained is around 30% for 10 −4 ppt (0.1 fg/ml) AFB1 which is greater than 1.5 times that of previous reports. This has been possible through the enhanced biomolecule immobilization capability of RGO. Thus the RGO based field effect capacitive sensor provides a combined advantage of both a high sensitivity and concentration dependent frequency behavior. [ABSTRACT FROM AUTHOR]

Abstract: Macroporous silicon is a promising material for wide spectrum of electrical applications ranging from photonic devices, microsystems engineering, supercapacitors to biosensors. Fabrication of good quality ohmic contact on macroporous silicon is a prerequisite for its widespread use in electrical sensor applications but it is also challenging since shallow nanopores are always present within the macropores. This paper aims at the formation of low resistance and fairly ohmic contacts on p-type macroporous silicon with 1.5μm pore diameter. Electrical contacts formed by vacuum evaporation of aluminium exhibit non-ohmic behaviour owing to the presence of the shallow nanoporous region formed on the surface of crystallites in macroporous silicon. To overcome this problem a controlled pre-annealing step at a temperature of 900°C for 30min leading to closure of shallow nanopores through coalescence of nanocrystallites of silicon due to grain growth has been carried out. The growth parameters are computed from the FESEM characteristics of the macroporous silicon preannealed at 900°C for varying time. For optimally preannealed macroporous silicon the I–V characteristics of macroporous silicon has been found to exhibit fairly ohmic behaviour with vacuum evaporated aluminium. Detailed electrical characterization of vacuum evaporated contacts on both unannealed and preannealed macroporous silicon has been carried out using modified TLM technique. It has been found that the specific contact resistance is almost five times lower for the latter. [Copyright &y& Elsevier]

• The impact of surface defects has been explored in terms of sensitivity. • Tuning of surface roughness and sensing of PSA have been done. • Transconductance of the sensors degrade monotonically with increasing defectdensity. • The optimized sensor has a PSA detection limit of 0.06 fM. • Nonoptimized sensor with initial detection limit of 0.1 fM has longevity of 4 months. In this work, the impact of surface defects in zinc oxide (ZnO) thin films on the performance of field effect transistor (FET) biosensors has been explored. Here, ZnO thin films have been fabricated using electron beam evaporation with various deposition parameters to tune the surface roughness and defect state density. The films have been deployed for sensing prostate specific antigen (PSA) after anti-PSA antibody immobilization. Interestingly, it has been observed that though the transconductance of the sensors degrade monotonically with increasing defect density, the antibody binding density has a non-monotonic behaviour due to enhanced steric hindrance effect beyond a certain limit of surface roughness. There is a variation of sensitivity by 15 times with different physical properties of ZnO and the optimized sensor has a PSA detection limit of 0.06 fM. Further, the sensor with the best sensitivity retains its performance upto 2 months while a non-optimized sensor with initial PSA detection limit of 0.1 fM has longevity of 4 months. Thus, it can be envisaged that tuning the surface defect state in ZnO FET biosensor is an essential requirement not only to ensure a low detection limit and high sensitivity but also to assure a reliable long term performance. [ABSTRACT FROM AUTHOR]