Soumyo Mukherji

Size-controlled silver nanoparticles synthesized over the range 5–100 nm using the same protocol and their antibacterial efficacy

A systematic and detailed study for size-specific antibacterial efficacy of silver nanoparticles (AgNPs) synthesized using a co-reduction approach is presented here. Nucleation and growth kinetics during the synthesis process was precisely controlled and AgNPs of average size 5, 7, 10, 15, 20, 30, 50, 63, 85, and 100 nm were synthesized with good yield and monodispersity. We found the bacteriostatic/bactericidal effect of AgNPs to be size and dose-dependent as determined by the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of silver nanoparticles against four bacterial strains. Out of the tested strains, Escherichia coli MTCC 443 and Staphylococcus aureus NCIM 5201 were found to be the most and least sensitive strains regardless of AgNP size. For AgNPs with less than 10 nm size, the antibacterial efficacy was significantly enhanced as revealed through delayed bacterial growth kinetics, corresponding MIC/MBC values and disk diffusion tests. AgNPs of the smallest size, i.e., 5 nm demonstrated the best results and mediated the fastest bactericidal activity against all the tested strains compared to AgNPs having 7 nm and 10 nm sizes at similar bacterial concentrations. TEM analysis of AgNP treated bacterial cells showed the presence of AgNPs on the cell membrane, and AgNPs internalized within the cells.

Novel U-bent fiber optic probe for localized surface plasmon resonance based biosensor.

The aim of this study is to develop an optical absorbance based biosensor suitable for wide scale use in resource-poor locales. A sensor for sensitive measurement of refractive index (RI) with the help of optical absorbance properties of gold nanoparticles (GNP) coupled to an efficient optical transducer in the form of a U-bent fiber optic probe is described. A U-bent probe was fabricated by a simple procedure. The absorbance due to the localized surface plasmon resonance (LSPR) of fiber-bound GNP was found to be linear to refractive index changes between 1.33 and 1.35. A U-bent probe of 200 microm diameter with a bend radius of 0.75 mm gave rise to a sensitivity of 35 DeltaA/RIU at 540 nm. The resolution of the sensor probe was 3.8x10(-5) RIU. Label-free biosensing was demonstrated using these probes with the help of IgG-anti IgG as bioreceptor-analyte pair.

Dendrimers in biosensors: Concept and applications

The performance of biosensors, i.e. the sensitivity, specificity, linearity, reusability, chemical stability, and reproducibility is critically dependent on the biofunctionalization of the sensor platform. The type(s) of linkers used for the immobilization of the capture probes and the exact immobilization protocol play a vital role in the overall performance of sensors. A variety of linker molecules have been used to biofunctionalize technologically important substrates (glass, gold, mica etc.). Amongst the different linkers, researchers have paid more attention to two dimensional architectures, e.g.silanes, polyaniline (PANI), alkanethiols, poly-L-lysine (PLL), etc. Despite extensive research and a large number of reports, researchers still face problems related to limited loading efficiency, limited accessibility of the probes, poor control over uniform spacing among the probes and a loss of functionality due to irregular orientation of the probes, all of which cause variability in the responses. Three dimensional gel based matrices have proved to be a better choice, except for the fact that the leaching of entrapped probe molecules has limited their use in developing sensor platforms. Taking into account the limitations of the two dimensional linker arrays and three dimensional gel matrices, supramolecular dendritic architectures have shown immense potential in designing and developing the sensor platforms. Dendrimers are well-defined, monodispersed, globular macromolecules constructed around a core unit. Different properties of dendrimers, i.e. their structural uniformity, globular shape, monodispersity, the existence of dendritic crevices, high functional group density, hydrophilicity, versatility to design dendrimer of different composition and their nanometric size can be exploited while developing high sensitivity biosensors. Researchers have demonstrated that these hyperbranched 3D molecules show enhanced sensitivity, reduced nonspecific binding, greater accessibility of the probe for the target analyte, high stability and low variability in their response. Hence, designing a sensor with a dendrimer as a linker is a successful approach to obtain superior sensor performance and minimize the overall cost of a sensor. In this article, we discuss various aspects of dendrimers from the point of view of sensor design, hoping that this review will excite more researchers into exploiting the exceptional properties of dendrimers in biosensor development.

Polymer nanocomposite nanomechanical cantilever sensors: material characterization, device development and application in explosive vapour detection

This paper reports an optimized and highly sensitive piezoresistive SU-8 nanocomposite microcantilever sensor and its application for detection of explosives in vapour phase. The optimization has been in improving its electrical, mechanical and transduction characteristics. We have achieved a better dispersion of carbon black (CB) in the SU-8/CB nanocomposite piezoresistor and arrived at an optimal range of 8-9 vol% CB concentration by performing a systematic mechanical and electrical characterization of polymer nanocomposites. Mechanical characterization of SU-8/CB nanocomposite thin films was performed using the nanoindentation technique with an appropriate substrate effect analysis. Piezoresistive microcantilevers having an optimum carbon black concentration were fabricated using a design aimed at surface stress measurements with reduced fabrication process complexity. The optimal range of 8-9 vol% CB concentration has resulted in an improved sensitivity, low device variability and low noise level. The resonant frequency and spring constant of the microcantilever were found to be 22 kHz and 0.4 N m(-1) respectively. The devices exhibited a surface stress sensitivity of 7.6 ppm (mN m(-1))(-1) and the noise characterization results support their suitability for biochemical sensing applications. This paper also reports the ability of the sensor in detecting TNT vapour concentration down to less than six parts per billion with a sensitivity of 1 mV/ppb.

Evanescent wave absorbance based fiber optic biosensor for label-free detection of E. coli at 280 nm wavelength

A novel label-free technique for the detection of pathogens based on evanescent wave absorbance (EWA) changes at 280 nm from a U-bent optical fiber sensor is demonstrated. Bending a decladded fiber into a U-shaped structure enhances the penetration depth of evanescent waves and hence sensitivity of the probe. We show that the enhanced EWA response from such U-bent probes, caused by the inherent optical absorbance properties of bacterial cells or biomolecules specifically bound to the sensor surface, can be exploited for the detection of pathogens. A portable optical set-up with a UV light emitting diode, a spectrometer and U-bent fiber optic probe of 200 μm core diameter, 0.75 mm bend radius and effective probe length of 1cm demonstrated an ability to detect less than 1000 cfu/ml.

Surface hydrophobicity of petroleum hydrocarbon degrading Burkholderia strains and their interactions with NAPLs and surfaces

Bacterial cell surface hydrophobicity (CSH) is an important factor governing the growth and adhesion behavior of microorganisms on non-aqueous phase liquids (NAPLs). In this work CSH and surface charge was quantified for three oil degrading Burkholderia cultures: aliphatic degrader Burkholderia cepacia (ES1) and two strains of aromatic degrading Burkholderia multivorans (NG1 and HN1) based on contact angle and zeta potential measurement. Model non-aqueous phase liquids (NAPLs) were formulated using n-hexadecane, naphthalene, phenanthrene and pyrene in varying concentration. Adhesion on to glass surfaces of varying hydrophobicity and adherence to n-hexadecane was quantified and correlated with hydrophobicity of the surface; variation in CSH of the culture in response to model NAPL used as growth substrate; and variation in zeta potential as a result of variation in growth substrate, ionic strength and pH of resuspension solution. B. cepacia (ES1) and B. multivorans (HN1) depicted comparable CSH which was higher than that of B. multivorans (NG1). For each culture, CSH was found to vary with the model NAPL used as growth substrate. Adhesion to glass increased with increase in CSH of the bacterial culture and with increase in hydrophobicity of the glass surface. B. cepacia (ES1) with lower negative zeta potential consistently depicted greater adhesion compared to B. multivorans (HN1). Adherence to n-hexadecane was significantly affected by various other factors, such as, growth substrate, pH, resuspension solution and their interactions as revealed through statistical analysis. These factors affected both the zeta potential and adherence to n-hexadecane to varying degree for the three Burkholderia cultures.

Microfabricated polymer chip with integrated U-bend waveguides for evanescent field absorption based detection

A mu-TAS system for evanescent field absorption with integrated polymer waveguides is reported for the first time. A photoresist SU-8 layer is patterned into a microchannel network, with U-bend waveguides and fiber-to-waveguide coupler structures. The aim of this study was to explore the possibility of using evanescent field absorption based sensing in conjunction with capillary electrophoresis for label free detection. We have proposed a novel design to couple the microchannel network with U-bend waveguides in a single step patterning of SU-8. In this novel design, the optical waveguide forms part of the microchannel wall, which aids in the detection process. The suitability of the device for optical applications was proved by absorbance measurement between 450 and 780 nm using Methylene Blue dye. Absorbance measurements were done by passing various concentrations of dye solutions through 200 microm and 500 microm microchannels. The device was also found sensitive to the refractive index (RI) of fluid flowing in the channel. The RI sensitivity was tested by passing sucrose solutions of varying concentrations through the channels and measuring absorbance across the integrated U-bend waveguides. The results indicate that such structures can be used easily for label free detection of molecules either by evanescent wave absorption or by changes associated with RI changes in the microenvironment around a waveguide.

Simple surface modification techniques for immobilization of biomolecules on SU-8

SU-8, an epoxy based negative photoresist polymer has found wide range of applications in the field of microfabrication based biosensors. SU-8 surfaces need to be modified in order to immobilize bioreceptors. We studied the possibility of grafting desired functional groups by means of simple chemical treatments under normal laboratory conditions. These chemical treatments involve the use of crosslinkers that are expected to react with epoxy groups or hydroxyl groups generated by acid/alkali treatment. Here, a comparison of the results obtained on surface modification using glycine and 11-mercapto undecanoic acid as crosslinkers is presented. Human Immunoglobin G (HIgG) was covalently immobilized to carboxylic acid on SU-8 surface using carbodiimide/succinimide chemistry. The activity of immobilized HIgG was verified by using fluorescence imaging of FITC tagged goat anti HIgG bound to the surface. Fluorescence imaging was used to determine the chemistry best suited to functionalize SU-8 surface for biosensor applications.

Evaluation of indigenous fresh water microalga Scenedesmus obtusus for feed and fuel applications: Effect of carbon dioxide, light and nutrient sources on growth and biochemical characteristics.

Scenedesmus obtusus, a freshwater microalga, was evaluated for its growth and biochemical characteristics under various culture conditions. S. obtusus was tolerant at all tested CO2 concentrations up to 20%. Among the different nitrogen sources, urea showed enhanced biomass productivities up to 2-fold compared to control, where the nitrogen source was sodium nitrate. Light intensity and photoperiod had a significant effect on growth rate and biomass productivity. The growth rate was observed maximum under continuous light exposure at the light intensities, 30μmolm(-2)sec(-1) and 60μmolm(-2)sec(-1) The species was able to tolerate the salinity levels up to 25mM NaCl, where, the increase in the concentration of NaCl suppressed the growth. Ammonium acetate and glycine showed better growth rate and biomass productivity indicating mixotrophic ability of S. obtusus. Supplementation of acetate and bicarbonate significantly enhanced the biomass productivity. Biodiesel properties of S. obtusus cultivated at various culture conditions were estimated.

“Organic CantiFET”: A Nanomechanical Polymer Cantilever Sensor With Integrated OFET

Nanomechanical cantilever based biochemical sensors translate molecular interactions into nanomechanical motions that can be measured by different transduction techniques. Improved sensitivity, reliability, and also cost effectiveness of such sensor platforms have been achieved by the use of polymer materials, along with the employment of smart and compatible transduction techniques. This paper explores an ultrasensitive nanomechanical cantilever sensor platform with a novel transduction technique by integrating a strain-sensitive organic field-effect transistor within a polymer nanomechanical cantilever. This sensor, named as “organic CantiFET,” has a surface stress sensitivity of 401 with a low-noise floor. This categorizes the organic CantiFET as an efficient biochemical sensor having a minimum detectable surface stress in the range of 0.18 mN/m.