Vincent M. Rotello

Gold Nanoparticles in Chemical and Biological Sensing

Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13 In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28 Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37 Figure 1 Physical properties of AuNPs and schematic illustration of an AuNP-based detection system. In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.

Gold nanoparticles in delivery applications

Gold nanoparticles (AuNPs) provide non-toxic carriers for drug and gene delivery applications. With these systems, the gold core imparts stability to the assembly, while the monolayer allows tuning of surface properties such as charge and hydrophobicity. An additional attractive feature of AuNPs is their interaction with thiols, providing an effective and selective means of controlled intracellular release.

Self-assembly of nanoparticles into structured spherical and networkaggregates

Multi-scale ordering of materials is central for the application of molecular systems in macroscopic devices. Self-assembly based on selective control of non-covalent interactions provides a powerful tool for the creation of structured systems at a molecular level, and application of this methodology to macromolecular systems provides a means for extending such structures to macroscopic length scale. Monolayer-functionalized nanoparticles can be made with a wide variety of metallic and non-metallic cores, providing a versatile building block for such approaches. Here we present a polymer-mediated ‘bricks and mortar’ strategy for the ordering of nanoparticles into structured assemblies. This methodology allows monolayer-protected gold particles to self-assemble into structured aggregates while thermally controlling their size and morphology. Using 2-nm gold particles as building blocks, we show that spherical aggregates of size 97 ± 17 nm can be produced at 23 °C, and that 0.5–1 µm spherical assemblies with (5–40) × 105 individual subunits form at -20 °C. Intriguingly, extended networks of ∼50-nm subunits are formed at 10 °C, illustrating the potential of our approach for the formation of diverse structural motifs such as wires and rods. These findings demonstrate that the assembly process provides control over the resulting aggregates, while the modularity of the ‘bricks and mortar’ approach allows combinatorial control over the constituents, providing a versatile route to new materials systems.

Detection and identification of proteins using nanoparticle–fluorescent polymer ‘chemical nose’ sensors

A sensor array containing six non-covalent gold nanoparticle-fluorescent polymer conjugates has been created to detect, identify and quantify protein targets. The polymer fluorescence is quenched by gold nanoparticles; the presence of proteins disrupts the nanoparticle-polymer interaction, producing distinct fluorescence response patterns. These patterns are highly repeatable and are characteristic for individual proteins at nanomolar concentrations, and can be quantitatively differentiated by linear discriminant analysis (LDA). Based on a training matrix generated at protein concentrations of an identical ultraviolet absorbance at 280 nm (A280 = 0.005), LDA, combined with ultraviolet measurements, has been successfully used to identify 52 unknown protein samples (seven different proteins) with an accuracy of 94.2%. This work demonstrates the construction of novel nanomaterial-based protein detector arrays with potential applications in medical diagnostics.

Wide varieties of cationic nanoparticles induce defects in supported lipid bilayers

Nanoparticles with widely varying physical properties and origins (spherical versus irregular, synthetic versus biological, organic versus inorganic, flexible versus rigid, small versus large) have been previously noted to translocate across the cell plasma membrane. We have employed atomic force microscopy to determine if the physical disruption of lipid membranes, formation of holes and/or thinned regions, is a common mechanism of interaction between these nanoparticles and lipids. It was found that a wide variety of nanoparticles, including a cell penetrating peptide (MSI-78), a protein (TAT), polycationic polymers (PAMAM dendrimers, pentanol-core PAMAM dendrons, polyethyleneimine, and diethylaminoethyl-dextran), and two inorganic particles (Au-NH2, SiO2-NH2), can induce disruption, including the formation of holes, membrane thinning, and/or membrane erosion, in supported lipid bilayers.

Tuning payload delivery in tumour cylindroids using gold nanoparticles.

Nanoparticles have great potential as controllable drug delivery vehicles because of their size and modular functionality. Timing and location are important parameters when optimizing nanoparticles for delivery of chemotherapeutics. Here we show that positively- and negatively-charged gold nanoparticles carrying either fluorescein or doxorubicin molecules move and localize differently in an in vitro three dimensional model of tumour tissue. Fluorescence microcopy and mathematical modelling showed that uptake, and not diffusion, is the dominant mechanism in particle delivery. Our results suggest that positive particles may be more effective for drug delivery because they are more significantly taken up by proliferating cells. Negative particles, which diffused faster, may perform better when delivering drugs deep into the tissues. An understanding of how surface charge can control tissue penetration and drug release may overcome some of the current limitations in drug delivery.

Surface functionalization of nanoparticles for nanomedicine

Control of interactions between nanoparticles and biosystems is essential for the effective utilization of these materials in biomedicine. A wide variety of nanoparticle surface structures have been developed for imaging, sensing, and delivery applications. In this research Highlight, we will emphasize advances in tailoring nanoparticle interfaces for implementation in nanomedicine.

Efficient Gene Delivery Vectors by Tuning the Surface Charge Density of Amino Acid-Functionalized Gold Nanoparticles

Gold colloids functionalized with amino acids provide a scaffold for effective DNA binding with subsequent condensation. Particles with lysine and lysine dendron functionality formed particularly compact complexes and provided highly efficient gene delivery without any observed cytotoxicity. Nanoparticles functionalized with first generation lysine dendrons (NP-LysG1) were approximately 28-fold superior to polylysine in reporter gene expression. These amino acid-based nanoparticles were responsive to intracellular glutathione levels, providing a tool for controlled release and concomitant expression of DNA.

Functionalized gold nanoparticles for drug delivery

Functionalized gold nanoparticles represent highly attractive and promising candidates in the applications of drug delivery owing to their unique dimensions, tunable functionalities on the surface and controllable drug release. This review illustrates the recent advances in the field of drug delivery using gold nanoparticles as carriers for therapeutic agents.

Effect of Nanoparticle Surface Charge at the Plasma Membrane and Beyond

Herein, we demonstrate that the surface charge of gold nanoparticles (AuNPs) plays a critical role in modulating membrane potential of different malignant and nonmalignant cell types and subsequent downstream intracellular events. The findings presented here describe a novel mechanism for cell-nanoparticle interactions and AuNP uptake: modulation of membrane potential and its effect on intracellular events. These studies will help understand the biology of cell-nanoparticle interactions and facilitate the engineering of nanoparticles for specific intracellular targets.