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The advent of nanostructures with localized surface plasmon resonance extending into the near infrared region has opened avenues for optical absorbance based sensors operating in this region. Sensing using the near infrared band has been known to confer many advantages, particularly for biosensing. Here we explore the basic refractive index sensing capabilities of such a sensor, made by coating gold nanoshells on a ‘U’ shaped optical fiber, as a prelude for its later application in biosensing.

Aspherical optical lenses are known for high performance and aberration free imaging. They are increasingly in demand for developing simple and compact high quality optical systems. The conventional methods of production of aspherical lenses are complex and time-consuming. There is a need for a simple, inexpensive and robust method of fabrication of such lenses. Here, we present a novel, low cost and simple approach to fabricate aspherical lenses reproducibly. The two-step process involves 1) controlled wetting of a curved surface with a transparent, cross-linkable polymer by dipping, and 2) pulling the wetted curved surface from the polymer solution. The curvature of the lens is dependent on the area of wetting, speed of pulling and the curvature of the curved surface. The lenses are produced with less than 5% error, and hence, the approach is reproducible in comparison to the previously reported techniques. A smartphone microscope developed using one of the fabricated lenses is found to have a resolution of ~1.7 μm. In addition, we show an application of these lenses as a means to check for the authenticity of Indian currency notes by a common man. The micro-patterns on the currency note are imaged using the smartphone microscope in ambient light. Also, the lenses have potential applications in developing compact and portable high quality optical systems, such as, endoscopes and microscopes.

We have demonstrated a fast, sensitive fluorescent tapered tip optical fiber (TTOF) sensor to detect copper (Cu (II)) ions. The principle of detection is based on fluorescent quenching of fluorescein isothiocynate (FITC) doped fluorescent silica nanoparticles (FSi) by Cu(II) ions. TTOF was functionalized with FITC doped fluorescent silica nanoparticles (FSi), as transduction element. Amine functionalized FSi nanoparticles are immobilized on aminosilanised tapered tip using glutaraldehyde treatment. Cu (II) ions which form chelation with amines quench the fluorescence (FL) of the FSi nanoparticles. Detection capacity down to 0.25 μM for copper ions has been achieved in this configuration.

Miniature lenses can transform commercial imaging systems, e.g., smartphones and webcams, into powerful, low-cost, handheld microscopes. To date, the reproducible fabrication of polymer lenses is still a challenge as they require controlled dispensing of viscous liquid. This paper reports a reproducible lens fabrication technique using liquid mold with programmable curvature and off-the-shelf materials. The lens curvature is controlled during fabrication by tuning the curvature of an interface of two immiscible liquids [polydimethylsiloxane (PDMS) and glycerol]. The curvature control is implemented using a visual feedback system, which includes a software-based guiding system to produce lenses of desired curvature. The technique allows PDMS lens fabrication of a wide range of sizes and focal lengths, within 20 min. The fabrication of two lens diameters: 1 and 5 mm with focal lengths ranging between 1.2 and 11 mm are demonstrated. The lens surface and bulk quality check performed using X-ray microtomography and atomic force microscopy reveal that the lenses are suitable for optical imaging. Furthermore, a smartphone microscope with similar to 1.4-mu m resolution is developed using a self-assembly of a single high power fabricated lens and microaperture. The lenses have various potential applications, e.g., optofluidics, diagnostics, forensics, and surveillance. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)

Hollow bimetallic nanostructures have recently emerged as attractive plasmonic materials due to the ease of optical tunability by changing their size/composition. Currently available methods, in addition to being tedious and time-consuming, result in polydispersed nanostructures, particularly due to polydispersed templates. In this study, optically tunable hollow gold nanostructures (HGNS) were synthesized by galvanic replacement reaction between silver nanospheres (AgNS) templates and gold salt. Monodispersed AgNS were created using a gold seed-mediated heteroepitaxial growth. Since it is easier to ensure monodispersed gold nanosphere seeds, the resulting AgNS showed a tight control on size. Hollow gold nanostructures 43-70 nm in size with extinction maxima ranging between 450-590 nm were produced by varying the gold to silver molar ratio. The nanostructures were observed to be monodispersed and uniform (SD

In this paper, we report a novel, eco-friendly method for the preparation of gold nanoshells (GNS) with unprecedented colloidal stability. Gold shell layers were grown on silica nanospheres by utilizing glucose. Nanoshell morphology was optimized by varying the molar ratio of glucose to gold, and was characterized using UV-vis spectroscopy and transmission electron microscopy (TEM). The colloidal stability of the prepared nanoshells was compared to those made using formaldehyde reductant, using sequential extinction intensity measurements and electron microscopy. Fourier transform infrared spectroscopy was used to elucidate their surface chemistry. Uniformity and homogeneity in the shells was achieved at a molar ratio of 2, followed by shell thinning at higher glucose concentrations. These colloids exhibited remarkable stability, compared to those prepared with the commonly reported protocol, where formaldehyde is employed as the reducing agent. The key role played by glucose in imparting high stability, in conjunction with its reducing properties is demonstrated. Furthermore, the sensing potential of these nanoshells was demonstrated using surface enhanced Raman scattering (SERS) in the near-infrared region on an optical fiber platform. The present approach offers an eco-friendly route for the production of nanoshells with high stability, augmenting their use for sensing and in vivo applications, where highly stable and unaggregated nanoshells are preferred. By eliminating the routinely used noxious formaldehyde, this method presents itself as a safe, scalable and direct route for the synthesis of glucose capped nanoshells, which are much sought after for therapeutic applications.

S-shaped optical fiber sensors were fabricated and its sensing potential was compared with U-bend sensors. Evanescent wave absorbance and refractive index sensitivity of S-bend were found to be 1.8-fold and 1.5-fold higher than U-bend.