Prasanna Kumar Mondal

DNA-Mediated Wirelike Clusters of Silver Nanoparticles: An Ultrasensitive SERS Substrate

Stable metal nanoclusters (NCs) with uniform interior nanogaps reproducibly offer a highly robust substrate for surface-enhanced Raman scattering (SERS) because of the presence of abundant hot spots on their surface. The synthesis of such an SERS substrate by a simple route is a challenging task. Here, we have synthesized a highly stable wirelike cluster of silver nanoparticles (Ag-NPs) with an interparticle gap of ~1.7 ± 0.2 nm using deoxyribonucleic acid (DNA) as the template by exploiting an easy and inexpensive chemical route. The red shift in the surface plasmon resonance (SPR) band of Ag-NCs compared to SPR of a single Ag-NP confirms the strong interplasmonic interaction. Methylene Blue (MB) is used as a representative Raman probe to study the SERS effect of the NCs. The SERS measurements reveal that uniform, reproducible, and strong Raman signals were observed up to the single-molecule level. The intensity of the Raman signal is not highly dependent on the polarization of the excitation laser. The DNA-based Ag-NCs as a substrate show better isotropic behavior for their SERS intensity compared to the dimer, as confirmed from both the experimental and theoretical simulation results. We believe that in the future the DNA-based Ag-NCs might be useful as a potential SERS substrate for ultrasensitive trace detection, biomolecular assays, NP-based photothermal therapeutics, and a few other technologically important fields.

An active device for volumetric measurement of drop nucleation in a superheated emulsion detector

An active volumetric device for active detection and measurement of drop nucleation of superheated emulsions is described. The drop nucleation is detected by an optical arrangement using a pair of a laser and a light-dependent resistor (LDR). The electric pulses from LDR are converted into binary pulses for counting. The device senses a volume change as low as 0.55 µL, and provides a fast data acquisition facility.

Study of gamma ray response of R404A superheated droplet detector using a two-state model

The superheated droplet detector (SDD) is known to be gamma ray insensitive below a threshold temperature which made them excellent candidates for neutron detection in the presence of gamma rays. Above the threshold temperature, the gamma ray detection efficiency increases with increase in temperature. In this work the gamma ray threshold temperature has been studied for SDD using R404A as the active liquid and is compared to the theoretical prediction. The temperature variation of gamma ray detection efficiency and interstate transition kinetics has also been studied using a two-state model. The experiments are performed at the ambient pressure of 1 atm and in the temperature range of 17-32 °C using a 662 keV (1)(37)Cs gamma ray source.

Characterization of R-134a superheated droplet detector for neutron detection

R-134a (C2H2F4) is a low cost, easily available and chlorine free refrigerant, which in its superheated state can be used as an efficient neutron detector. Due to its high solubility in water the R-134a based superheated droplet detectors (SDD) are usually very unstable unless the detector is fabricated using a suitable additive, which stabilizes the detector. The SDD is known to have superheated droplets distributed in a short-lived and in a relatively long-lived metastable states. We have studied the detector response to neutrons using a (241)AmBe neutron source and obtained the temperature variation of the nucleation parameters and the interstate kinetics of these droplets using a two-state model.

A new method for measurement of droplet size distribution in superheated emulsions

Superheated emulsion contains micron-sized superheated liquid droplets suspended in a host liquid medium. The superheated droplets nucleate to vapor bubbles when energetic radiation deposits a sufficient amount of energy within a very small localized region of the active liquid. Droplet nucleation is associated with a change in volume and also emission of an acoustic pulse, both of which can be detected electronically. When exposed to energetic radiation, the metastable superheated droplets nucleate independently of each other, and the number of drops nucleated and volume of vapor formed due to nucleation of these droplets are both acquired simultaneously as a function of time. These two data are then fitted simultaneously to obtain the droplet size distribution of the emulsion. The size distributions of R-114 (C2Cl2F4)- and R-12 (CCl2F2)-based emulsions are obtained by this method and compared with direct optical measurements.

Study of acoustic emission due to vaporisation of superheated droplets at higher pressure

Abstract Bubble nucleation in superheated liquids can be controlled by adjusting the ambient pressure and temperature. At higher pressure the threshold energy for bubble nucleation increases, and we have observed that the amplitude of the acoustic emission during vaporisation of superheated droplet decreases with increase in pressure at any given temperature. Other acoustic parameters such as the primary harmonic frequency and the decay time constant of the acoustic signal also decrease with increase in pressure. This behavior is independent of the type of superheated liquid. The decrease in signal amplitude limits the detection of bubble nucleation at higher pressure. This effect is explained by the emission of shockwave generated during the supersonic growth of the microbubble in superheated liquids.

The gamma ray detection threshold temperature of different superheated droplet detectors

The response of superheated droplet detectors (SDD) loaded with four different active liquids, R-12 (CCl2F2), R-114 (C2Cl2F4), R-134a (C2H2F4) and R-610 (C4F10) have been studied to obtain the gamma ray detection threshold temperature (Tγ) of the respective detectors. A 137Cs gamma ray source is used for this study. To obtain Tγ from the experimental data a phenomenological model has been used. Result indicates that Tγ maintains a linear relationship with the limit of superheat (Tlim) of different active liquids used in SDD. It indicates that the limit of superheat of a liquid can be used for the prediction of its gamma ray detection threshold temperature.

Note: A new optical method for the detection of bubble nucleation in superheated droplet detector

A superheated droplet detector (SDD) consists of a large number of micron-sized superheated liquid drops suspended in a gel medium. The vaporization of a superheated drop is associated with the emission of an acoustic signal. A novel optical method is developed for the detection of this acoustic signal. In this method, a probe-bubble picks up the acoustic signal, and the oscillation of the probe-bubble is detected by employing a laser and phototransistor. The method can detect vaporization of an individual superheated drop in real-time and can be used for studying the response of SDDs to ionizing radiations.