Jitendra Satija

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.

Emerging use of nanostructure films containing capped gold nanoparticles in biosensors

The localized surface plasmon resonance (LSPR) property of gold nanoparticles (GNP) has been exploited in a variety of optical sensor configurations including solution-based bioassays, paper-based colorimetric detection, surface-confined nanoparticle film/array-based sensing, etc. Amongst these, gold nanostructured films are of great interest because of their high stability, good reproducibility, robustness, and cost-effectiveness. The inherent optical characteristics of GNP, are attributed to parameters like size and shape (eg, nanospheres, nanorods, nanostars), eg, LSPR spectral location sensitivity to the local environment, composition (eg, gold-silver or silica-gold nanoshells), sensing volume, mesospacing, and multiplexing. These properties allow sensor tunability, enabling enhanced sensitivity and better performance of these biosensors. Ultrasensitive biosensor designs were realized using gold nanostructured films fabricated by bottom-up as well as top-down approaches. In this review, we describe the past, present, and future trends in the development of GNP-LSPR-based sensors, concentrating on both design (fabrication) and application. In the process, we have discussed various combinations of GNP size and shape, substrate, and application domains.

Plasmonic-ELISA: expanding horizons

The enzyme-linked immunosorbent assay (ELISA) has been an integral part of in vitro diagnostic tests due to its high specificity, standard configuration, and convenient readout. The convergence of the plasmonic property, i.e. localized surface plasmon resonance (LSPR), of noble metal nanoparticles with the ELISA technique has established a novel category of immunoassay, known as “plasmonic ELISA” (pELISA). It has enabled the naked eye detection of various disease biomarkers down to attomolar concentration. The color development relies on the biocatalytic reaction mediated modulation of LSPR properties. Through this review we present various current state-of-art pELISA strategies adopted for naked-eye quantification of disease biomarkers along with their advantages, limitations and applicability. These strategies are broadly classified into following four categories based on the different means of LSPR modulation: (i) aggregation, (ii) controlled growth kinetics, (iii) metallization, and (iv) etching of metal nanoparticles. Furthermore, the paper highlights the different biocatalytic reactions and their role in plasmonic color development as a function of analyte concentration. The review ends with an elucidation of current challenges and future perspectives of pELISA precisely focusing on the need for development of next generation low cost point-of-care diagnostic kits.

Evanescent Wave Absorption Based Fiber-Optic Sensor - Cascading of Bend and Tapered Geometry for Enhanced Sensitivity

Evanescent wave absorption (EWA) based fiber-optic sensors have found widespread applications ranging from environmental sensing to biosensing. In these sensors, optical and geometrical characteristics such as optical fiber type (single-mode or multi-mode), fiber core diameter, fiber probe geometry, fiber probe length, etc., are very important. These parameters affect the penetration depth and fractional power by modulating the ray propagating in the fiber probe that ultimately influences the sensitivity of the EWA sensors. Various geometries of fiber probe designs, like bent, tapered, coiled, etc., have been explored for improving the sensitivity. This chapter describes the design, development and fabrication of a novel bent-tapered fiber-optic sensor. A combination of bending and tapering acts as a mode converter, which results in high penetration depth of the evanescent field. In addition, tapered region of the probe increases the coupling efficiency at the detector end by V-number matching and thus improves the signal-to-noise ratio. EWA sensitivity of the sensor was compared for different taper ratios. Finally, the optimized geometrical design was used to demonstrate biosensing application.

Glucose mediated synthesis of gold nanoshells: A facile and eco-friendly approach conferring high colloidal stability

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.

A dendrimer matrix for performance enhancement of evanescent wave absorption-based fiber-optic biosensors

Dendrimers are monodispersed, hyperbranched macromolecules which have a globular shape and high pendant group density. Due to these unique properties, dendrimers have been used for catalytic, biochemical, pharmaceutical, biomedical and energy transfer applications. This study demonstrates the potential of a dendrimeric biointerface for the immobilization of bioreceptors, and the overall improvement of the performance of evanescent wave absorbance-based fiber-optic biosensors. Amine functionalized sensor substrates obtained by fourth generation poly(amidoamine) dendrimer immobilization were compared with substrates obtained by a conventional silanization technique that used aminopropyl triethoxysilane. A fluorescein isothiocyanate-based amine assay and X-ray photoelectron spectroscopy showed that the dendrimer matrix had 2.4 times the number of amine groups of the silanized surface. Further, atomic force microscopy-based topographic analysis revealed that the dendrimer matrix caused a 1.3-fold increase in surface area compared to the silane matrix, which caused a negligible change. In addition to a two-fold improvement in antibody loading compared to conventionally functionalized probes, an eight-fold increase in the sensor response for critically important lower concentrations of analyte was observed with dendrimer coated U-bent fiber-optic probes. Our detailed experimental studies demonstrated the potential of the dendrimer for preparing a biosensing platform with high protein loading, amplified sensor response and low nonspecific protein binding.

Facile synthesis of size and wavelength tunable hollow gold nanostructures for the development of a LSPR based label-free fiber-optic biosensor

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 ≤ 11%) in all the batches. Furthermore, the synthesized HGNS were immobilized on dendrimer-functionalized U-shaped fiber-optic probes to develop a localized surface plasmon resonance (LSPR) based sensor. Refractive index sensitivity of the HGNS based sensors was found to be 1.5-fold higher than solid gold nanosphere (GNS)-based fiber-optic sensors. These HGNS-based fiber-optic probes were subsequently used to develop an immunosensor with improved sensitivity by using human immunoglobulin-G (HIgG) as receptor molecules and goat-anti-HIgG as a target analyte.

Etching-based transformation of dumbbell-shaped gold nanorods facilitated by hexavalent chromium and their possible application as a plasmonic sensor

The seed-mediated synthesis of anisotropic gold nanorods (AuNRs) has attracted attention due to their tunable morphology-dependent optical properties and wide range of applicability. Since the growth of nanorods can be modulated by metal ions, we have explored the Cr(VI)-assisted transformation of AuNRs. In the current investigation, the transformation of dumbbell-shaped AuNRs by Cr(VI) has been studied based on observations from UV-visible spectroscopy, transmission electron microscopy, mean hydrodynamic size measurements, and zeta potential analyses. The Cr(VI)-assisted concentration-dependent reshaping of dumbbell-shaped nanorods to shorter nanorods and spherical particles was observed with a corresponding change in their spectral properties, rod length, and zeta potential. A mechanism to understand this reshaping and etching of dumbbell-shaped nanorods into smooth rods is also proposed. The application of dumbbell-shaped AuNRs for Cr(VI) detection has been presented based on the reshaping effect observed. This method offers a detection limit of 0.071 μM with linearity in the range of 2–10 μM (R2 = 0.9978). This is the first-ever study, wherein the concentration-dependent transition of dumbbell-shaped AuNRs upon interaction with Cr(VI) was extensively investigated. This method displays good sensitivity and selectivity against most interferents and has been validated in environmental samples (lake, tap, and bore well water) with high recovery rates.

Dendrimeric nano-glue material for localized surface plasmon resonance-based fiber-optic sensors

In this study, we have investigated dendrimeric architecture as “nano-glue” material for RI-sensitive fiber-optic sensors. Dendrimers are immobilized on fiber-optic probes using a simple method that includes dipping, rinsing and drying of probes at room temperature. Dendrimer binding was confirmed by contact angle measurement and fluorescein isothiocyanate binding studies. These functionalized probes were coated with gold nanoparticles to develop localized surface plasmon resonance-based refractive index sensor. RI sensitivity measurement revealed that the dendrimeric matrix enhanced the RI sensitivity by 1.4-fold compared to two-dimensional amino-silanized sensor matrices. This suggests that dendrimer molecules are better choice as “nano-glue” material for fiber-optic sensors.

A novel 'Gold on Gold' biosensing scheme for an on-fiber immunoassay

In this paper, we propose a novel „gold on gold‟ biosensing scheme for absorbance based fiber-optic biosensor. First, a self-assembled monolayer of gold nanoparticles is formed at the sensing region of the fiber-optic probe by incubating an amino-silanized probe in a colloidal gold solution. Thereafter, the receptor moieties, i.e. Human immunoglobulin G (HIgG) were immobilized by using standard alkanethiol and classic carbodiimide coupling chemistry. Finally, biosensing experiments were performed with different concentrations of gold nanoparticle-tagged analyte, i.e. Goat anti- Human immunoglobulin G (Nanogold-GaHIgG). The sensor response was observed to be more than five-fold compared to the control bioassay, in which the sensor matrix was devoid of gold nanoparticle film. Also, the response was found to be ~10 times higher compared to the FITC-tagged scheme and ~14.5 times better compared to untagged scheme. This novel scheme also demonstrated the potential in improving the limit of detection for the fiber-optic biosensors.