Stimuli-responsive SERS nanoparticles: conformational control of plasmonic coupling and surface Raman enhancement

Flower-shaped gold nanoparticles: synthesis, characterization and their application as SERS-active tags inside living cells

The detection of Raman signals inside living cells is a topic of great interest in the study of cell biology mechanisms and for diagnostic and therapeutic applications. This work presents the synthesis and characterization of flower-shaped gold nanoparticles and demonstrates their applicability as SERS-active tags for cellular spectral detection. The particles were synthesized by a facile, rapid new route that uses ascorbic acid as a reducing agent of gold salt. Two triarylmethane dyes which are widely used as biological stains, namely malachite green oxalate and basic fuchsin, were used as Raman-active molecules and the polymer mPEG-SH as capping material. The as-prepared SERS-active nanoparticles were tested on a human retinal pigment epithelial cell line and found to present a low level of cytotoxicity and high chemical stability together with SERS sensitivity down to picomolar particle concentrations.

Hybrid micro-/nanogels for optical sensing and intracellular imaging

Hybrid micro-/nanogels are playing an increasing important part in a diverse range of applications, due to their tunable dimensions, large surface area, stable interior network structure, and a very short response time. We review recent advances and challenges in the developments of hybrid micro-/nanogels toward applications for optical sensing of pH, temperature, glucose, ions, and other species as well as for intracellular imaging. Due to their unique advantages, hybrid micro-/nanogels as optical probes are attracting substantial interests for continuous monitoring of chemical parameters in complex samples such as blood and bioreactor fluids, in chemical research and industry, and in food quality control. In particular, their intracellular probing ability enables the monitoring of the biochemistry and biophysics of live cells over time and space, thus contributing to the explanation of intricate biological processes and the development of novel diagnoses. Unlike most other probes, hybrid micro-/nanogels could also combine other multiple functions into a single probe. The rational design of hybrid micro-/nanogels will not only improve the probing applications as desirable, but also implement their applications in new arenas. With ongoing rapid advances in bionanotechnology, the well-designed hybrid micro-/nanogel probes will be able to provide simultaneous sensing, imaging diagnosis, and therapy toward clinical applications.

Highly sensitive and selective dynamic light-scattering assay for TNT detection using p-ATP attached gold nanoparticle

TNT is one of the most commonly used nitro aromatic explosives for landmines of military and terrorist activities. As a result, there is an urgent need for rapid and reliable methods for the detection of trace amount of TNT for screenings in airport, analysis of forensic samples, and environmental analysis. Driven by the need to detect trace amounts of TNT from environmental samples, this article demonstrates a label-free, highly selective, and ultrasensitive para-aminothiophenol (p-ATP) modified gold nanoparticle based dynamic light scattering (DLS) probe for TNT recognition in 100 pico molar (pM) level from ethanol:acetonitile mixture solution. Because of the formation of strong π-donor-acceptor interaction between TNT and p-ATP, para-aminothiophenol attached gold nanoparticles undergo aggregation in the presence of TNT, which changes the DLS intensity tremendously. A detailed mechanism for significant DLS intensity change has been discussed. Our experimental results show that TNT can be detected quickly and accurately without any dye tagging in 100 pM level with excellent discrimination against other nitro compounds.

Thermo-induced electromagnetic coupling in gold/polymer hybrid plasmonic structures probed by surface-enhanced raman scattering

This paper describes a general stepwise strategy combining diazonium salt, surface-initiated atom transfer radical polymerization (SI-ATRP), and click chemistry for an efficient gold surface functionalization by poly(N-isopropylacrylamide) (PNIPAM) brushes and gold nanoparticle assemblies. We designed by this way a new plasmonic device made of gold nanoparticles separated from a gold film through a thermoresponsive polymer layer. This organic layer responds to temperature variations by conformational changes (with a characteristic temperature called the lower critical solution temperature, LCST) and is therefore able to vary the distance between the gold nanoparticles and the gold film. The optical properties of these stimulable substrates were probed by surface-enhanced raman scattering (SERS) using methylene blue (MB) as a molecular probe. We show that an increase of the external temperature reversibly induces a significant enhancement of the MB SERS signal. This was attributed to a stronger interaction between the gold nanoparticles and the gold substrate. The temperature-responsive plasmonic devices developed in this paper thus provide a dynamic SERS platform, with thermally switchable electromagnetic coupling between the gold nanoparticles and the gold surface.

Interface design and multiplexed analysis with surface plasmon resonance (SPR) spectroscopy and SPR imaging

Ever since the advent of surface plasmon resonance (SPR) and SPR imaging (SPRi) in the early 1990s, their use in biomolecular interaction analysis (BIA) has expanded phenomenally. An important research area in SPR sensor development is the design of novel and effective interfaces that allow for the probing of a variety of chemical and biological interactions in a highly selective and sensitive manner. A well-designed and robust interface is a necessity to obtain both accurate and pertinent biological information. This review covers the recent research efforts in this area with a specific focus towards biointerfaces, new materials for SPR biosensing, and novel array designs for SPR imaging. Perspectives on the challenges ahead and next steps for SPR technology are discussed.

SERS-active gold lace nanoshells with built-in hotspots

Development of multifunctional drug delivery vehicles with therapeutic and imaging capabilities as well as in situ methods of monitoring of intracellular processes will greatly benefit from a simple method of preparation of plasmonic Au structures with nanometer scale gaps between sharp metallic elements where the so-called SERS hot spots can be formed. Here the synthesis of gold lace capsules with average diameters ca. 100 nm made of a network of metallic branches 3-5 nm wide and separated by 1-3 nm gaps is reported. Biocompatible amphiphilic polyurethanes (PUs) were used as template for these particles. The unusual topology of the produced gold lace shells somewhat reminiscent of Fabergé eggs is likely to reflect the network of hydrophobic and hydrophilic domains of PU globules. The gold lace develops from initial open weblike structures by gradual enveloping the PU template. The diameter of gold lace shell is determined by the size of PUs in water and can be adjusted by the molecular mass of PUs. The close proximity between branches makes them excellent supports for surface-enhanced Raman spectroscopy (SERS), which was demonstrated using 1-naphthalenethiol upon excitation with photons with different wavelengths. The loading and releasing of pyrene as a model of hydrophobic drugs and the use of SERS to monitor it were demonstrated.

Multifunctional hybrid nanogel for integration of optical glucose sensing and self-regulated insulin release at physiological pH

Optical detection of glucose, high drug loading capacity, and self-regulated drug delivery are simultaneously possible using a multifunctional hybrid nanogel particle under a rational design in a colloid chemistry method. Such hybrid nanogels are made of Ag nanoparticle (NP) cores covered by a copolymer gel shell of poly(4-vinylphenylboronic acid-co-2-(dimethylamino)ethyl acrylate) [p(VPBA-DMAEA)]. The introduction of the glucose sensitive p(VPBA-DMAEA) gel shell onto Ag NPs makes the polymer-bound Ag NPs responsive to glucose. While the small sized Ag cores (10 +/- 3 nm) provide fluorescence as an optical code, the responsive polymer gel shell can adapt to a surrounding medium of different glucose concentrations over a clinically relevant range (0-30 mM), convert the disruptions in homeostasis of glucose level into optical signals, and regulate release of preloaded insulin. This shows a new proof-of-concept for diabetes treatment that exploits the properties from each building block of a multifunctional nano-object. The highly versatile multifunctional hybrid nanogels could potentially be used for simultaneous optical diagnosis, self-regulated therapy, and monitoring of the response to treatment.

Gold nanoparticles functionalized with block copolymers displaying either LCST or UCST thermosensitivity in aqueous solution

Gold nanoparticles (AuNPs) coated with a diblock copolymer composed of poly(ethylene oxide) (PEO) and poly(N,N-dimethylaminoethyl methacrylate) (PEO-b-PDMAEMA) were prepared, while their reaction with 1,3-propane sultone gave rise to quaternized PEO-b-PDMAEMA chains on AuNPs. Using pyrene as a probe equilibrated with polymer-coated AuNP aqueous solutions, the fluorescence measurement results show that the diblock copolymers tethered to AuNP surfaces could undergo a thermally induced solubility change characterized by either a LCST (with PEO-b-PDMAEMA) or a UCST (with quaternized PEO-b-PDMAEMA). In both solutions, the use of a water-soluble PEO outer block helps preventing aggregation of AuNPs upon dehydration (decreased solubility) of either PDMAEMA chains at T > LCST or quaternized PDMAEMA chains at T < UCST. Moreover, the reversible solubility change of block copolymer chains on AuNPs could be induced not only by heating the bulk solution but also, to some extent, by illuminating the solution at wavelengths close to the surface plasmon resonance (SPR) band of AuNPs due to a photothermal effect. The method presented is an easy way to prepare polymer-coated AuNPs that can have a stable dispersion in aqueous solution while exhibiting either LCST or UCST thermosensitivity.

Using polymers to photoswitch the aggregation state of gold nanoparticles in aqueous solution

Gold nanoparticles (AuNPs) coated with a water-soluble polymer displaying a photoswitchable lower critical solution temperature (LCST) were synthesized, and the photoinduced change in the aggregation state of AuNPs was investigated. The photoactivity of AuNPs was conferred by tethering on the surface a random copolymer of N,N-dimethylacrylamide and N-4-phenylazophenyl acrylamide (P(DMA-co-Azo)), whose LCST could be lowered upon the trans-cis photoisomerization of azobenzene groups. It was found that the dispersion of P(DMA-co-Azo)-coated AuNPs in aqueous solution remained stable regardless of temperature and the isomeric form of azobenzene groups. However, with the presence of free P(DMA-co-Azo) chains in the solution, the reversible photoinduced shift of LCST of the polymer could result in a reversible photoswitching between dispersion and aggregation of AuNPs. A stable dispersion could be obtained at T < LCST of the polymer with azobenzene in the trans form (without irradiation), while upon UV irradiation, the lowering of LCST with cis-azobenzene put the solution to above the altered LCST and the insoluble polymer chains led to aggregation of AuNPs; the initial dispersion state could be recovered by visible light irradiation, bringing cis-azobenzene back to the trans form. The use of such a photoresponsive polymer to optically tune the aggregation state of AuNPs allows the surface plasmon resonance (SPR) of AuNPs to be reversibly switched by light at two wavelengths, the extent of which depends on the amount of free polymer chains in the solution.

Surface-enhanced Raman scattering biomedical applications of plasmonic colloidal particles

This review article presents a general view of the recent progress in the fast developing area of surface-enhanced Raman scattering spectroscopy as an analytical tool for the detection and identification of molecular species in very small concentrations, with a particular focus on potential applications in the biomedical area. We start with a brief overview of the relevant concepts related to the choice of plasmonic nanostructures for the design of suitable substrates, their implementation into more complex materials that allow generalization of the method and detection of a wide variety of (bio)molecules and the strategies that can be used for both direct and indirect sensing. In relation to indirect sensing, we devote the final section to a description of SERS-encoded particles, which have found wide application in biomedicine (among other fields), since they are expected to face challenges such as multiplexing and high-throughput screening.