Amartya Sengupta

Graphene-based wireless bacteria detection on tooth enamel

Direct interfacing of nanosensors onto biomaterials could impact health quality monitoring and adaptive threat detection. Graphene is capable of highly sensitive analyte detection due to its nanoscale nature. Here we show that graphene can be printed onto water-soluble silk. This in turn permits intimate biotransfer of graphene nanosensors onto biomaterials, including tooth enamel. The result is a fully biointerfaced sensing platform, which can be tuned to detect target analytes. For example, via self-assembly of antimicrobial peptides onto graphene, we show bioselective detection of bacteria at single-cell levels. Incorporation of a resonant coil eliminates the need for onboard power and external connections. Combining these elements yields two-tiered interfacing of peptide-graphene nanosensors with biomaterials. In particular, we demonstrate integration onto a tooth for remote monitoring of respiration and bacteria detection in saliva. Overall, this strategy of interfacing graphene nanosensors with biomaterials represents a versatile approach for ubiquitous detection of biochemical targets.

Carbon Dioxide Carbonates in the Earth’s Mantle: Implications to the Deep Carbon Cycle

An increase in the ionic character in C-O bonds at high pressures and temperatures is shown by the chemical/phase transformation diagram of CO{sub 2}. The presence of carbonate carbon dioxide (i-CO{sub 2}) near the Earths core-mantle boundary condition provides insights into both the deep carbon cycle and the transport of atmospheric CO{sub 2} to anhydrous silicates in the mantle and iron core.

Phase diagram of carbon dioxide: update and challenges

We present the phase diagram of carbon dioxide with the most recent finding of coesite-like carbon dioxide, a missing analog to SiO2, address several controversies on phase VII and phase IV in terms of the phase metastabilities and thermal path-dependent phase transitions, and discuss the implications to the generalized phase diagram of simple molecular solids.

THz reflection spectroscopy of C-4 explosive and its detection through interferometric imaging

In recent times, Terahertz (1 THz = 1012 cycles/sec and 300 μm in wavelength) spectroscopy has become a promising technique for spectroscopic identification of different materials having contemporary interest. In this study we report a direct measurement of reflection spectra of the explosive C-4, which shows significant absorption around 0.8 THz, using THz time domain spectroscopic techniques. A contrast in reflection of around 8% has also been observed between the neighboring frequencies of 0.7 THz and 0.9 THz. The spectral data have been used to create realistic synthetic images for use in simulations of interferometric detection in a stand-off THz imaging system. The results obtained are analyzed using Artificial Neural Networks for positive identification of the agents with an interferometric array of few linear detectors in near field mode.

Terahertz interferometric imaging of a concealed object

Experimental results of two-dimensional homodyne terahertz interferometric imaging are presented. The performance of an N element detector array is imitated by only one detector placed at N positions. Continuous waves at 0.25-0.3 THz are used to detect concealed objects: a metal object and an RDX sample. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.

Tunable optical properties in atomic layer deposition grown ZnO thin films

ZnO thin films with very low surface roughness and bulklike electron density were grown on Si and SiO2 by atomic layer deposition. The real and imaginary parts of the complex dielectric function of ZnO on Si show monotonically decreasing values with decreasing film thickness at and below a threshold of about 20 nm. On the other hand, x-ray reflectivity results show that the electron density of our ZnO films is close to that of bulk ZnO and does not vary considerably with film thickness. While the reduction of the dielectric function cannot be explained by the electron density of our ZnO films, the Tanguy–Elliott amplitude prefactor governing the strength of optical interband transitions can explain our results consistently through the lowering of the electron–hole overlap factor at the ZnO/Si interface. In the case of ZnO/Si, a staggered type-II (spatially indirect) quantum well, holes are scattered into the Si substrate, causing a lowering of the electron–hole overlap factor and thus the reduction of exc...

Grain size dependent scattering studies of common materials using THz time domain techniques

In recent times, the far infrared or the terahertz (1 THz = 1012 cycles/sec and 300μm in wavelength) region of electromagnetic spectrum has become a promising radiation for spectroscopic identification of different types of biomaterials. The present work investigates the effect of grain size on the THz spectra of chalk, salt, sugar and flour using THz time-domain spectroscopy. It has been observed that at lower frequencies, solids of small grain sizes of nonabsorbing materials show rising trends in their extinction spectra. Here, we obtain extinction spectra of granular salt, chalk, sugar and flour between 0.2 to 1.2 THz and show that the experimentally obtained extinction can be predicted on the basis of the Mie Scattering model for small grain sizes. The current study is an attempt to understand the absorption spectrum of a few such materials having no significant intrinsic absorption in the THz region by separating the independent contributions of true absorption of the material and scattering losses due to its morphology in the extinction of the material. This would help in distinguishing these materials based on their rising trend of the extinction spectra at lower frequencies.

Transparent Large Anvil Press for In-situ Raman and Laser Heating

It has been shown that high-pressure polymorphs of simple materials often display novel thermal/mechanical/optical properties and can be recovered at ambient conditions with significant stability (or metastability). Therefore, high-pressure synthesis of such materials offers an avenue toward novel materials applications. However, the present high-pressure method of diamond-anvil cell (DAC) or large volume press (LVP) has a fairly limited use in this regard because of a small amount of sample or the lack of in-situ optical diagnostic, respectively. Hybridizing the merits of DAC and LVP, we have recently developed a simple yet unique device, called a Transparent Large Anvil Press (TLAP), which consists of an opposed diamond anvil cell and a Paris-Edinburgh press. Coupling with a wide range of optical diagnostics such as Raman spectroscopy and laser heating, the TLAP is capable of in-situ investigation and synthesis of a milligram quantity of cryogenic samples at high pressures and temperatures.

Terahertz interferometric and synthetic aperture imaging

Experimental results of homodyne terahertz interferometric 1-D and 2-D imaging are presented. Continuous waves at 0.25-0.3 THz are used to detect a metal object behind a barrier. The performance of an N element detector array is imitated by only one detector placed at N positions. The reconstructed images are in a good agreement with theoretical predictions. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.

TERAHERTZ INTERFEROMETRIC AND SYNTHETIC APERTURE IMAGING

Experimental results of homodyne terahertz interferometric 1-D and 2-D imaging are presented. The reconstructed images of a point source are in a good agreement with theoretical predictions. The performance of an N element detector array is imitated by only one detector placed at N positions. Continuous waves at 0.25-0.3 THz are used to detect a metal object behind a barrier. 1-D images of a C-4 sample have been obtained at several terahertz frequencies. Focusing issues of 2-D imaging have been demonstrated. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.