Nano-biophotonics is an emerging field created by the fusion of photonics, nanotechnology and biomedical sciences. An interdisciplinary team comprising physicists, biologists, biotechnologists and chemists are applying their skills towards development of novel techniques for in vitro disease diagnosis and treatment.
(1) Surface Enhanced Raman Spectroscopy (SERS)
Surface enhanced Raman spectroscopy (SERS) is a technique where the Raman signal gets considerably enhanced owing to the electromagnetic field enhancement due to localized surface plasmon resonances. Metal nanoparticles and specialized substrates for SERS are generally fabricated with metals (mainly gold and silver) that support the plasmonic resonance.
SERS substrates may be in the form of nanoparticle colloid, aggregated nanoparticles in solution, nanoparticles assembled on a surface, drop casted nanoparticles, laser direct writing, or structured surface with ordered nanoparticle arrays. A vast body of literature has shown that SERS can lead to significant improvement in the prospects of Raman spectroscopy for detecting molecular signatures in trace amounts of analytes. We, at LBAD, have also explored the potential of SERS in detection of disease specific analytes in body fluids. It was found that SERS signal of creatinine at concentration of 100 mg/dl is significantly enhanced with respect to the convention Raman signal at 500 mg/dl. However, we found that despite its immense potential for disease diagnosis using body fluids, on-field clinical application of SERS is limited by two major factors: (i) large spatial variability in the measured Raman intensities due to differential (and often random) distribution of surface ‘hotspots’ and (ii) lack of reproducibility. We recently developed a novel technique, Nano-Trap Enhanced Raman Spectroscopy (NTERS) for overcoming these long-standing limitations and challenges of SERS.
(2) Nano-Trap Enhanced Raman Spectroscopy (NTERS)
NTERS combines the Drop-Coating Deposition (DCD), metal nanoparticle trapping and the nanoparticle based SERS to result in Raman signal enhancement that is much superior to any of these techniques. The technique relies on the formation of “hotspots” upon drying up of a micro volume drop of the liquid containing an aqueous mixture of nanoparticles and analytes on an aluminium foil substrate in the presence of a focused Raman excitation laser beam.
The Raman signal is simultaneously detected from these “hotspots”. To elaborate, as the micro volume drop of liquid containing metal nanoparticles and the analyte starts drying, both the analyte molecules and the nanoparticles move towards the edge of the drop as a result of the ‘coffee ring effect’. The laser beam is focused at a point on the periphery of this drying drop. This focused laser beam creates two forces on a nanoparticle; gradient and scattering. As the wavelength of the laser is away from the absorption band of the nanoparticle, the gradient force dominates over the scattering force, and the nanoparticle is pushed towards the focus of the beam, hence, it is trapped. Due to trapping, the nanoparticles tend to form aggregates on the surface, at the laser focal point. As this point is at the periphery of the drying drop, analyte molecules too tend to accumulate (coffee ring effect) and this leads to caging of analyte molecules within the nanoparticle aggregates, and thus resulting in formation of “hotspots”. The measurement of Raman spectra from these spots results in significantly higher Raman signal due to the electromagnetic and chemical enhancement phenomenon similar to SERS. Further, since the laser mediated drying of the sample drop results in more uniform aggregation, it leads to high reproducibility of the measured NTERS signal. The results of ours studies showed that as compared to the conventional SERS and DCDRS, NTERS yielded significantly better (around two orders of magnitude) signal enhancement as well as reproducibility. The other significant advantage is that the technique is simple and cost effective as it does not require, unlike SERS, preparation of any specialized substrate.
