Core-shell Fe3O4@Au nanocomposite as dual-functional optical probe and potential removal system for arsenic (III) from Water.

We report for the very first time, development of a dual functional nanocomposite to perform as an optical probe as well as removal system for As(III) from ground water. Upon suitable thiolation using dithiothreitol (DTT), the Fe3O4(core)-Au(shell) nanocomposite (DTT-Fe3O4@Au) has been fabricated that can detect As(III) in aqueous solution with significantly low limit of detection and holds potential for selective removal of As(III) from water owing to its magnetic core. Due to high affinity of -SH groups for As(III), the nanoparticles undergo aggregation in the presence of As(III), resulting in a significant decrease in absorbance, yielding the limit of detection (LOD) as 0.86 ppb, which is much lower than the World Health Organisation (WHO) recommended threshold value of 10 ppb. UV-vis spectroscopy in conjunction with dynamic light scattering and electron microscopy techniques have further elaborated the detailed interaction between As(III) and DTT-Fe3O4@Au nanocomposite regarding their size dynamics and solution behaviour during the interaction. Moreover, ca.70% removal of As(III) from aqueous solution by DTT-Fe3O4@Au has been observed by ICP-MS measurement strengthening the potential of this nanocomposite as a dual-functional probe and filter.

A chemical quenching- and physical blocking-based method to minimize process-mediated aggregation of antibody-crosslinked nanoparticles for imaging application.

Critical limitation of nanoparticles (NP) is their aggregation after functionalisation and antibody cross-linking. We analysed the cause of this aggregation with respect to functionalities (carboxyls and amines) on the NP surface. We have devised a low cost novel method to reduce such aggregations during protein cross-linking and validated it by probing the platelet surface with platelet surface-specific anti-CD41 antibody conjugated NPs.

Dendrimer driven self-assembly of SPR active silver-gold nanohybrids.

A fourth generation PAMAM dendrimer has been successfully employed for the development of a single step synthesis strategy for self-assembled Ag-Au nanohybrid structures. The surface plasmon resonance properties and the degree of self-assembly of the nanohybrid are strongly correlated with the stoichiometry of the metals which gives rise to enhanced plasmonic properties. The enhanced plasmonic response of the nanohybrids is modeled and is validated experimentally in a model HRP (horseradish peroxidise) bioassay carried out on an SPR-based biochip platform.

Synthesis and characterization of model silica-gold core-shell nanohybrid systems to demonstrate plasmonic enhancement of fluorescence.

In this work, gold-silica plasmonic nanohybrids have been synthesized as model systems which enable tuning of dye fluorescence enhancement/quenching interactions. For each system, a dye-doped silica core is surrounded by a 15 nm spacer region, which in turn is surrounded by gold nanoparticles (GNPs). The GNPs are either covalently conjugated via mercapto silanization to the spacer or encapsulated in a separate external silica shell. The intermediate spacer region can be either dye doped or left undoped to enable quenching and plasmonic enhancement effects respectively. The study indicates that there is a larger enhancement effect when GNPs are encapsulated in the outer shell compared to the system of external conjugation. This is due to the environmental shielding provided by shell encapsulation compared to the exposure of the GNPs to the solvent environment for the externally conjugated system. The fluorescence signal enhancement of the nanohybrid systems was evaluated using a standard HRP-anti-HRP fluorescence based assay platform.

Synthesis and characterization of a Noble metal Enhanced Optical Nanohybrid (NEON): a high brightness detection platform based on a dye-doped silica nanoparticle.

A highly bright and photostable, fluorescent nanohybrid particle is presented which consists of gold nanoparticles (GNPs) embedded in dye-doped silica in a core-shell configuration. The dye used is the near-infrared emitting 4,5-benzo-5'-(iodoacetaminomethyl)-1',3,3,3',3'-pentamethyl-1-(4-sulfobutyl) indodicarbo cyanine. The nanohybrid architecture comprises a GNP core which is separated from a layer of dye molecules by a 15 nm buffer layer and has an outer protective, undoped silica shell. Using this architecture, a brightness factor of 550 has been achieved compared to the free dye. This hybrid system, referred to as Noble metal Enhanced Optical Nanohybrid (NEON) in this paper, is the first nanohybrid construct to our knowledge which demonstrates such tunable fluorescence property. NEON has enhanced photostability compared to the free dye and compared to a control particle without GNPs. Furthermore, the NEON particle, when used as a fluorescent label in a model bioassay, shows improved performance over assays using a conventional single dye molecule label.

Label-free optical characterization methods for detecting amine silanization-driven gold nanoparticle self-assembly.

Fluorescence lifetime correlation spectroscopy (FLCS) is presented as a single-step label-free detection method for probing the amine silanization-driven spontaneous 3D self-assembly of freestanding gold nanoparticles (GNPs) in solution. Unlike the conventional methods of studying self-assembly, for example, UV-vis spectroscopy and electron microscopy, FLCS utilizes the intrinsic gold fluorescence. The significance of this approach is to amalgamate the measurement of optical and hydrodynamic size properties simultaneously to achieve a more coherent description of the self-assembly pathway. GNP self-assembly has two-stage kinetics. Electrostatic interaction drives the initial amine silanization, and this is followed by siloxane bond formation between hydrolyzed ethoxy groups of GNP-attached APTES, resulting in the formation of micrometer-sized superstructures. The self-assembly has resulted in a 5-fold increase in the fluorescence lifetime (FL), and the FLCS study has shown an 8- to 10-fold increase in the diffusion coefficient using the pure diffusion model. This result is consistent with the transmission electron microscopy (TEM) observation, which shows a few hundred fold increase in the diameter due to assembly formation by the GNPs.

Novel multiparametric approach to elucidate the surface amine-silanization reaction profile on fluorescent silica nanoparticles.

This Article addresses the important issue of the characterization of surface functional groups for optical bioassay applications. We use a model system consisting of spherical dye-doped silica nanoparticles (NPs) that have been functionalized with amine groups whereby the encapsulated cyanine-based near-infrared dye fluorescence acts as a probe of the NP surface environment. This facilitates the identification of the optimum deposition parameters for the formation of a stable ordered amine monolayer and also elucidates the functionalization profile of the amine-silanization process. Specifically, we use a novel approach where the techniques of fluorescence correlation spectroscopy (FCS) and fluorescence lifetime measurement (FL) are used in conjunction with the more conventional analytical techniques of zeta potential measurement and Fourier transfer infrared spectroscopy (FTIR). The dynamics of the ordering of the amine layer in different stages of the reaction have been characterized by FTIR, FL, and FCS. The results indicate an optimum reaction time for the formation of a stable amine layer, which is optimized for further biomolecular conjugation, whereas extended reaction times lead to a disordered cross-linked layer. The results have been validated using an immunoglobulin (IgG) plate-based direct binding assay where the maximum number of IgG-conjugated aminated NPs were captured by immobilized anti-IgG antibodies for the NP sample corresponding to the optimized amine-silanization condition. Importantly, these results point to the potential of FCS and FL as useful analytical tools in diverse fields such as characterization of surface functionalization.

Fluorescence lifetime analysis and fluorescence correlation spectroscopy elucidate the internal architecture of fluorescent silica nanoparticles.

Fluorescence lifetime (FL) analysis and fluorescence correlation spectroscopy (FCS) have been successfully employed to reveal detailed information about the internal architecture of fluorescent silica nanoparticles (NPs). The dual-component lifetime behavior shows a two-domain dye distribution in the NP as a function of solvent accessibility. The introduction of an undoped silica shell serves to stabilize the outer dye fraction that is manifest as an increase in lifetime. The FCS not only shows a size-dependent increase in the cross-correlation decay constant but also demonstrates a significant relaxation in the FCS signal with the introduction of an undoped silica shell.

Thermal hysteresis of some important physical properties of nanoparticles.

Gold nanoparticles show thermal hysteresis with properties such as surface plasmon absorption, conductivity, and zeta potential. The direction of the incremental change in plasmon peak position and its extinction depend on the nature of surface conjugation. The thermal profile of a surface plasmon resonance spectrum for nanoparticles may serve as a signature for the associated small molecule or macromolecule on which it is seeded. The thermal responses of zeta potential and conductivity profile are found to be independent of the surface conjugation with the later being subjected to a phase transition phenomenon as revealed by a temperature criticality.

Controllable self-assembly from fibrinogen-gold (fibrinogen-Au) and thrombin-silver (thrombin-Ag) nanoparticle interaction.

Fibrinogen conjugated gold nanoparticles (fibrinogen-Au) and thrombin conjugated silver nanoparticles (thrombin-Ag) were synthesized by heating (90 degrees C) the proteins (50 microg protein/ml) with 1mM AgNO(3) or AuCl(3). The resultant particles were harvested and examined by flow cytometry, scanning electron microscopy (SEM), transmission emission microscopy (TEM), optical microscopy and dynamic light scattering. SEM and TEM images revealed that the fibrinogen-Au and thrombin-Ag particles interacted. The emergent bio-nanoconjugate population could be controlled by addition of thrombin-Ag. The method may be exploited in parametrizing coagulation factors and other clinically important protein-protein interactions.

Compensatory secondary structure alterations in protein glycation.

Glycation, a local covalent interaction, leads to alterations in secondary and tertiary structures of hemoglobin, the changes produced by fructose being more pronounced than those caused by glucose. The Stokes diameter of hemoglobin increases upon glycation from 7 to 14 nm and a concurrent inter-chain cross-linking and heme loss are also observed, particularly in the later stage of glycation. An initial increase of tryptophan (trp) fluorescence was observed in both glucation and fructation. In case of frucation however there was a decrease in tryptophan fluorescence that was accompanied by an increase in fluorescence of the advanced glycosylation end products (AGEs). This fluorescence behavior is indicative of energy transfer between tryptophan and the AGEs formed during the late stage of glycation. Emergence of an isosbestic point in the fluorescence spectra (taken at different time intervals) implies existence of two distinct glycation stages. The late glycation stage is also marked by an increase of beta structure and random coil at the expense of alpha helix. It is further observed that this compensatory loss of alpha helix (reported for the first time) and increase in beta sheet and random coil elements depend on the number of solvent-accessible glycation sites (rather than total number of such sites) and the subunit assembly of the protein.

Potential of cadmium sulphide nanorods as an optical microscopic probe to the folding state of cytochrome C.

The folding behavior of cytochrome C (Cyt-C) conjugated with CdS nanorods (CdSnr) is amenable to monitoring by bright field microscopy, the porosity and percolating behavior of such protein conjugated nanoclusters depending on the folding history prior to the conjugation. The method has been used to predict the thermal melting behavior as well as guanidine hydrochloride induced unfolding of Cyt-C. Dynamic light scattering studies indicate that the size distribution of the nanoforms widens in presence of the protein. Furthermore, there is emergence of clusters with higher conductivity and altered zeta potential. Increase of second virial coefficient of CdS nanoforms in the presence of Cyt-C (obtained from static light scattering experiments) implies presence of protein coat over the hydrophobic nanosurface. The results are supported by morphological changes observed through scanning electron microscopy (SEM). Accordingly, the X-ray diffraction pattern shows a change of crystallographic orientations of CdSnr in presence of Cyt-C.

A size dependent folding contour for cytochrome C.

The paper describes an experimental construct of the folding route of the heme protein cytochrome-C. The construct highlights a slowing down near the nose of the folding funnel caused by the multiplicity of the energy traps near the native conformation created as a result of complex heme-peptide interaction. Interestingly the hydrodynamic size, the size heterogeneity and peroxidase activity serve as a triple measure of the distance of this near equilibrium departure from native conformation. Accordingly, the folding process is marked with a gradual and reversible reduction of mean hydrodynamic size, size heterogeneity and peroxidase activity (higher in unfolded state). The Dynamic Light Scattering based straightforward illustration of hydrodynamic size variation may serve as a model to slow folding observed in case of heme proteins, the heme itself serving as a natural facilitator for the native peptide conformation.