Shramik Sengupta

Ultrafine sputter-deposited Pt nanoparticles for triiodide reduction in dye-sensitized solar cells: impact of nanoparticle size, crystallinity and surface coverage on catalytic activity

This paper presents a detailed electrochemical impedance spectroscopy and cyclic voltammetry (CV) investigation into the electrocatalytic activity of ultrafine (i.e., smaller than 2 nm) platinum (Pt) nanoparticles generated on a fluorine-doped tin oxide (FTO) surface via room temperature tilted target sputter deposition. In particular, the Pt-decorated FTO electrode surfaces were tested as counter electrode candidates for triiodide (I3(-)) reduction in dye-sensitized solar cells (DSSCs). We observed a direct correlation between size-dependent Pt nanoparticle crystallinity and the I3(-) reduction activity underlying DSSC performance. CV analysis confirmed the higher electrocatalytic activities of sputter-deposited crystalline Pt nanoparticles (1-2 nm) compared with either sub-nanometre Pt clusters or a continuous Pt thin film. While the low catalytic activity and DSSC performance of Pt clusters smaller in size than 1 nm is believed to arise from their non-crystalline nature and charge-trapping attributes, we attribute the high catalytic performance of larger Pt nanoparticles in the 1-2 nm regime to their well-defined crystallinity and fast electron transfer kinetics. For DSSC applications, the optimized Pt loading was calculated to be ~2.54 × 10(-7) g cm(-2), which corresponds to surface coverage by ~1.6 nm sized Pt nanoparticles.

Rapid on-chip genetic detection microfluidic platform for real world applications

The development of genetic detection protocols for field applications is an important aspect of modern medical diagnostic technology and environmental monitoring. In this paper, we report a rapid, portable, and inexpensive DNA hybridization technique using a bead-based microfluidic platform that functions by passing fluorescently labeled target DNA through a chamber packed with functionalized beads within a microfluidic channel. DNA hybridization is then assessed using a digital camera attached to a Clare Chemical DR-45M dark reader non-UV transilluminator that uses visible light as an excitation source and a blue and amber filter to reveal fluorescence. This microfluidic approach significantly enhances hybridization by reducing the diffusion time between target DNA and the silica surface. The use of probe-functionalized beads as solid support also enhances the sensitivity and limit of detection due to a larger surface area per unit volume. This platform could be adapted for use in medical applications and environmental monitoring, including the detection of harmful organisms in the ballast water of ships.

Coatings and surface modifications imparting antimicrobial activity to orthopedic implants

Bacterial colonization and biofilm formation on an orthopedic implant surface is one of the worst possible outcomes of orthopedic intervention in terms of both patient prognosis and healthcare costs. Making the problem even more vexing is the fact that infections are often caused by events beyond the control of the operating surgeon and may manifest weeks to months after the initial surgery. Herein, we review the costs and consequences of implant infection as well as the methods of prevention and management. In particular, we focus on coatings and other forms of implant surface modification in a manner that imparts some antimicrobial benefit to the implant device. Such coatings can be classified generally based on their mode of action: surface adhesion prevention, bactericidal, antimicrobial-eluting, osseointegration promotion, and combinations of the above. Despite several advances in the efficacy of these antimicrobial methods, a remaining major challenge is ensuring retention of the antimicrobial activity over a period of months to years postoperation, an issue that has so far been inadequately addressed. Finally, we provide an overview of additional figures of merit that will determine whether a given antimicrobial surface modification warrants adoption for clinical use.

Capture of circulating tumor cells using photoacoustic flowmetry and two phase flow

Melanoma is the deadliest form of skin cancer, yet current diagnostic methods are unable to detect early onset of metastatic disease. Patients must wait until macroscopic secondary tumors form before malignancy can be diagnosed and treatment prescribed. Detection of cells that have broken off the original tumor and travel through the blood or lymph system can provide data for diagnosing and monitoring metastatic disease. By irradiating enriched blood samples spiked with cultured melanoma cells with nanosecond duration laser light, we induced photoacoustic responses in the pigmented cells. Thus, we can detect and enumerate melanoma cells in blood samples to demonstrate a paradigm for a photoacoustic flow cytometer. Furthermore, we capture the melanoma cells using microfluidic two phase flow, a technique that separates a continuous flow into alternating microslugs of air and blood cell suspension. Each slug of blood cells is tested for the presence of melanoma. Slugs that are positive for melanoma, indicated by photoacoustic waves, are separated from the cytometer for further purification and isolation of the melanoma cell. In this paper, we evaluate the two phase photoacoustic flow cytometer for its ability to detect and capture metastatic melanoma cells in blood.

Novel Electrical Method for Early Detection of Viable Bacteria in Blood Cultures

ABSTRACT We present a novel electrical method for detecting viable bacteria in blood cultures that is 4 to 10 times faster than continuous monitoring blood culture systems (CMBCS) like the Bactec system. Proliferating bacteria are detected via an increase in the bulk capacitance of suspensions, and the threshold concentration for detection is ∼104 CFU/ml (compared to ∼108 CFU/ml for the Bactec system).

Ionic conductivity enhancement of sputtered gold nanoparticle-in-ionic liquid electrolytes

Ionic liquids (ILs) are being widely investigated as advanced electrolytes within electric double-layer capacitors (EDLCs) due to their inherent ionic conductivity, wide electrochemical windows, essentially zero volatility, and high temperature stability. Despite being composed entirely of ions, the ionic conductivity of a typical IL is significantly hindered by its high viscosity, rendering it akin to normal electrolytes. In this light, in order to increase the applicability of IL electrolytes, it is of the utmost priority to discover approaches for improving the electrochemical properties of ILs without adversely affecting their other beneficial attributes. In this work, we make important strides toward this goal by employing low energy sputtering to generate novel electrolytes comprising gold nanoparticle dispersions within the prototypical IL 1-ethyl-3-methylimidazolium ethyl sulfate, [emim][EtSO4]. This study also afforded the unique opportunity to investigate nanoscale growth mechanisms occurring within the IL. Cyclic voltammetry and electrochemical impedance spectroscopy analyses revealed that when the IL contained a substantial fraction of sub-nanometer-sized particles, the double-layer capacitance was increased by ∼190%, concomitant with a bulk electrolyte resistance decrease of ∼70% with respect to a gold-free control. An exponential rise in resistance accompanied by a proportional decrease in capacitance accompanies nanoparticle growth until a critical size is reached—typically within 10 h at room temperature—beyond which the final capacitance is typically ∼60% higher than the control with an electrolyte resistance similar to the control. Overall, our results reveal an anomalous capacitance increase and low internal resistance for nanoparticle-in-IL dispersions, suggesting intriguing potential as electrolytes for next-generation EDLCs, fuel cells, and sensors.

Novel Electrical Method for the Rapid Determination of Minimum Inhibitory Concentration (MIC) and Assay of Bactericidal/BacteriostaticActivity

We present a rapid (4-hr) electrical method for Antibiotic Susceptibility Testing that not only yields the MIC of candidate antibiotics, but also simultaneously determines the antibiotics’ effect on the bacteria (bactericidal/bacteriostatic). Unlike conventional “impedance microbiology” methods that rely on measuring the effects of bacterial metabolism on the conductance/impedance of the suspension at a single chosen frequency, our method uses measurements at 500 frequencies between 1 KHz and 100 MHz to estimate the amount of electric charge stored due to charge-polarization at intact cell-membranes of living bacteria (the suspension “bulk capacitance”). By doing so, we are able to track the number of live bacteria in suspensions as the observations are taken (every 1 hour). It thus determines whether the numbers of viable bacteria present is increasing (bacteria proliferating in presence of antibiotic), decreasing (bacteria being killed) or holding steady (bacterial numbers held static). Three well-characterized bacterial strains (E. coli ATCC- 25922, S. aureus ATCC-29213 and P. aeruginosa ATCC-27853) were tested against a range of concentrations (0 to 128 mg/l) of known static and cidal antibiotics. For each sample (bacterial strain at a given concentration of antibiotic), statistical analysis of the “bulk capacitance” values, recorded over 4 hours was used to determine whether the bacteria were proliferating, being killed, or being held static. The minimum concentration of antibiotic for which the bacteria were killed or failed to proliferate is considered the Minimum Inhibitory Concentration (MIC). MICs obtained fell within the expected range for the strains tested, and “static” and “cidal” antibiotics were correctly identified. This method thus demonstrates the potential to provide in 4 hrs, clinically relevant information such as the MIC of bacterial strains (that currently take up to 2 days) and the mode of action (static/cidal) that currently takes an additional day.

Kinetically limited differential centrifugation as an inexpensive and readily available alternative to centrifugal elutriation.

When separating two species with similar densities but differing sedimentation velocities (because of differences in size), centrifugal elutriation is generally the method of choice. However, a major drawback to this approach is the requirement for specialized equipment. Here, we present a new method that achieves similar separations using standard benchtop centrifuges by loading the seperands as a layer on top of a dense buffer of a specified length, and running the benchtop centrifugation process for a calculated amount of time, thereby ensuring that all faster moving species are collected at the bottom, while all slower moving species remain in the buffer. We demonstrate the use of our procedure to isolate bacteria from blood culture broth (a mixture of bacterial growth media, blood, and bacteria).

Detection and Isolation of Circulating Melanoma Cells using Photoacoustic Flowmetry

Circulating tumor cells (CTCs) are those cells that have separated from a macroscopic tumor and spread through the blood and lymph systems to seed secondary tumors(1,2,3). CTCs are indicators of metastatic disease and their detection in blood samples may be used to diagnose cancer and monitor a patients response to therapy. Since CTCs are rare, comprising about one tumor cell among billions of normal blood cells in advanced cancer patients, their detection and enumeration is a difficult task. We exploit the presence of pigment in most melanoma cells to generate photoacoustic, or laser induced ultrasonic waves in a custom flow cytometer for detection of circulating melanoma cells (CMCs)(4,5). This process entails separating a whole blood sample using centrifugation and obtaining the white blood cell layer. If present in whole blood, CMCs will separate with the white blood cells due to similar density. These cells are resuspended in phosphate buffered saline (PBS) and introduced into the flowmeter. Rather than a continuous flow of the blood cell suspension, we induced two phase flow in order to capture these cells for further study. In two phase flow, two immiscible liquids in a microfluidic system meet at a junction and form alternating slugs of liquid(6,7). PBS suspended white blood cells and air form microliter slugs that are sequentially irradiated with laser light. The addition of a surfactant to the liquid phase allows uniform slug formation and the user can create different sized slugs by altering the flow rates of the two phases. Slugs of air and slugs of PBS with white blood cells contain no light absorbers and hence, do not produce photoacoustic waves. However, slugs of white blood cells that contain even single CMCs absorb laser light and produce high frequency acoustic waves. These slugs that generate photoacoustic waves are sequestered and collected for cytochemical staining for verification of CMCs.

Quantitative assessment of reflux in commercially available needle-free IV connectors:

Introduction: Blood reflux is caused by changes in pressure within intravascular catheters upon connection or disconnection of a syringe or intravenous tubing from a needle-free connector (NFC). Changes in pressure, differing with each brand of NFC, may result in fluid movement and blood reflux that can contribute to intraluminal catheter occlusions and increase the potential for central-line associated bloodstream infections (CLABSI). Methods: In this study, 14 NFC brands representing each of the four market-categories of NFCs were selected for evaluation of fluid movement occurring during connection and disconnection of a syringe. Study objectives were to 1) theoretically estimate amount of blood reflux volume in microliters (μL) permitted by each NFC based on exact component measurements, and 2) experimentally measure NFC volume of fluid movement for disconnection reflux of negative, neutral and anti-reflux NFC and fluid movement for connection reflux of positive displacement NFC. Results: The results demonstrated fluid movement/reflux volumes of 9.73 μL to 50.34 μL for negative displacement, 3.60 μL to 10.80 μL for neutral displacement, and 0.02 μL to 1.73 μL for pressure-activated anti-reflux NFC. Separate experiment was performed measuring connection reflux of 18.23 μL to 38.83 μL for positive displacement NFC connectors. Conclusions: This study revealed significant differences in reflux volumes for fluid displacement based on NFC design. While more research is needed on effects of blood reflux in catheters and NFCs, results highlight the need to consider NFCs based on performance of individual connector designs, rather than manufacturer designation of positive, negative and neutral marketing categories for NFCs without anti-reflux mechanisms.