Sudeshna Pal

A Multichannel Femtoampere-Sensitivity Potentiostat Array for Biosensing Applications

Rapid and accurate detection of pathogens using conductometric biosensors requires potentiostats that can measure small variations in conductance. In this paper, we present an architecture and implementation of a multichannel potentiostat array based on a novel semi-synchronous sigma-delta (SigmaDelta) analog-to-digital conversion algorithm. The algorithm combines continuous time SigmaDelta with time-encoding machines, and enables measurement of currents down to femtoampere range. A 3-mmtimes3-mm chip implementing a 42-channel potentiostat array has been prototyped in a 0.5-mum CMOS technology. Measured results demonstrate that the prototype can achieve 10 bits of resolution, with a sensitivity down to 50-fA current. The power consumption of the potentiostat has been measured to be 11 muW per channel for a sampling rate of 250 kHz. Experiments with a conductometric biosensor specific to Bacillus Cereus bacterium, demonstrate the ability of the potentiostat in identifying different concentration levels of the pathogen in a biological sample

Silicon photonic crystal nanocavity-coupled waveguides for error-corrected optical biosensing.

A photonic crystal (PhC) waveguide based optical biosensor capable of label-free and error-corrected sensing was investigated in this study. The detection principle of the biosensor involved shifts in the resonant mode wavelength of nanocavities coupled to the silicon PhC waveguide due to changes in ambient refractive index. The optical characteristics of the nanocavity structure were predicted by FDTD theoretical methods. The device was fabricated using standard nanolithography and reactive-ion-etching techniques. Experimental results showed that the structure had a refractive index sensitivity of 10(-2) RIU. The biosensing capability of the nanocavity sensor was tested by detecting human IgG molecules. The device sensitivity was found to be 2.3±0.24×10(5) nm/M with an achievable lowest detection limit of 1.5 fg for human IgG molecules. Additionally, experimental results demonstrated that the PhC devices were specific in IgG detection and provided concentration-dependent responses consistent with Langmuir behavior. The PhC devices manifest outstanding potential as microscale label-free error-correcting sensors, and may have future utility as ultrasensitive multiplex devices.

Selective Virus Detection in Complex Sample Matrices with Photonic Crystal Optical Cavities

Rapid, sensitive, and selective detection of viruses is critical for applications in medical diagnostics, biosecurity, and environmental safety. In this article, we report the application of a point-defect-coupled W1 photonic crystal (PhC) waveguide biosensor to label-free optical detection of viruses. Fabricated on a silicon-on-insulator (SOI) substrate using electron-beam (e-beam) lithography and reactive-ion-etching, the PhC sensing platform allows optical detection based on resonant mode shifts in response to ambient refractive index changes produced by infiltration of target biomaterial within the holes of the PhC structure. Finite difference time domain (FDTD) calculations were performed to assist with design of the sensor, and to serve as a theoretical benchmark against which experimental results could be compared. Using Human Papillomavirus virus-like particles (VLPs) spiked in 10% fetal bovine serum as a model system, we observed a limit of detection of 1.5 nM in simple (buffer only) or complex (10% serum) sample matrices. The use of anti-VLP antibodies specific for intact VLPs with the PhC sensors provided highly selective VLP detection.

Electrically active magnetic nanoparticles as novel concentrator and electrochemical redox transducer in Bacillus anthracis DNA detection

Magnetic polymer nanostructures are a new class of multifunctional nanomaterials that are recently being explored in biosensor devices. In this paper, for the first time we report the novel application of electrically active magnetic (EAM) nanoparticles as concentrator of DNA targets as well as electrochemical transducers for detection of the Bacillus anthracis protective antigen A (pag A) gene. The EAM nanoparticles are synthesized by chemical polymerization and have dimensions of 80-100 nm. The biosensor detection encompasses two sets of DNA probes that are specific to the target gene: the detector probe labeled with the EAM nanoparticles and the biotinylated capture probe. The DNA targets are double hybridized to the detector and the capture probes and concentrated from nonspecific DNA fragments by applying a magnetic field. Subsequently, the DNA sandwiched targets (EAM-detector probe-DNA target-capture probe-biotin) are captured on streptavidin modified screen printed carbon electrodes through the biotinylated capture probes. Detection is achieved electrochemically by measuring the oxidation-reduction signal of the EAM nanoparticles. Preliminary results indicate that the biosensor is able to detect the redox signal of the EAM nanoparticles at DNA concentrations as low as 0.01 ng/μl.

Spatio-Temporal Processing for Multichannel Biosensors Using Support Vector Machines

Rapid-response biosensing systems are necessary to counteract threats due to foreign and high-consequence pathogens. A yes/no multichannel biosensor is an important tool that enables simultaneous detection of different pathogens, independent of their relative concentration level. This paper proposes a novel multichannel biosensing technique, which combines multiclass support vector machines (SVMs) with multichannel immunosensors. The method combines spatial and temporal information generated by the multichannel immunosensor for rapid and reliable discrimination between pathogens of interest. This paper demonstrates that by including temporal and cross-reactive spatial signatures, the accuracy of the system can be improved at low pathogen concentration levels and for discrimination between closely related strains of pathogens. Compensation of systematic and biosensor fabrication errors is achieved by the use of a supervised SVM training which is also used in system calibration. Experimental results, with a prototype multichannel biosensor used for discriminating strains of E. coli (K12 and O157 : H7) and Salmonella enterica serovar Thompson, show an accuracy of 98% for concentration levels, 100-108 colony forming units per milliliter, and total detection time of less than 6 min

Microcavities in photonic crystal waveguides for biosensor applications

In this study, resonant microcavities in photonic crystal (PhC) waveguides are investigated for biosensing applications. The device architecture consists of a PhC waveguide with a defect line for guiding the transmission of light. Resonant microcavities created by changing the radius of a hole adjacent to the defect line are coupled to the PhC waveguide. Detection is based on shifts in the resonance wavelength observed in the transmission spectra. The PhC waveguide device is fabricated on silicon-on-insulator (SOI) wafers using electron beam lithography and reactive-ion etching (RIE). Receptor molecules are attached to the defects in the device by standard amino-silane and glutaraldehyde crosslinking chemistry. Preliminary results demonstrate successful detection of human IgG molecules as the target at large concentration levels of 500 μg/ml. Such PhC waveguide devices are advantageous for medical diagnostics and biosecurity applications as they allow rapid, label-free, and sensitive detection of multiple analytes in a single platform.

Novel thermal model aided motor derating under waveform distortion

Static power converters and nonlinear loads used by industries change the sinusoidal nature of power system voltages, resulting in the injection of harmonic voltages in the power system. Due to the presence of harmonics, the total losses of the motor will increase, resulting in a higher temperature rise of motor which may not be allowed due to the faster deterioration of the motor insulating materials, thereby reducing the life-expectancy of the motor. This paper reports on the development of a novel hybrid thermal model, based on which the temperature distributions along the motor cross-section under different operating conditions can be obtained. Utilising this thermal model, the derating of the motor can be carried out more effectively.

Nanocavities in Photonic Crystal Waveguides for Label-Free Biosensing

We have investigated resonant nanocavities coupled to photonic crystal waveguides for biosensing. The devices are fabricated using electron beam lithography and reactive-ion-etching. Preliminary results demonstrate successful detection of human IgG molecules and refractive index sensing.