Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

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.

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Gold nanoparticles in chemical and biological sensing.

Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Nanostructured plasmonic sensors.

Colloidal nanoparticles are being intensely pursued in current nanoscience research. Nanochemists are often frustrated by the well-known fact that no two nanoparticles are the same, which precludes the deep understanding of many fundamental properties of colloidal nanoparticles in which the total structures (core plus surface) must be known. Therefore, controlling nanoparticles with atomic precision and solving their total structures have long been major dreams for nanochemists. Recently, these goals are partially fulfilled in the case of gold nanoparticles, at least in the ultrasmall size regime (1–3 nm in diameter, often called nanoclusters). This review summarizes the major progress in the field, including the principles that permit atomically precise synthesis, new types of atomic structures, and unique physical and chemical properties of atomically precise nanoparticles, as well as exciting opportunities for nanochemists to understand very fundamental science of colloidal nanoparticles (such as the s...

Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities

9.2. Protein-Based Hg(II) Detectors 3467 9.3. Oligonucleotide-Based Hg(II) Detector 3467 9.4. DNAzyme-Based Hg(II) Detectors 3469 9.5. Antibody-Based Hg(II) Detector 3469 10. Mercury Detectors Based on Materials 3469 10.1. Soluble and Fluorescent Polymers 3469 10.2. Membranes, Films, and Fibers 3471 10.3. Micelles 3473 10.4. Nanoparticles 3473 11. Perspectives 3474 12. Addendum 3475 12.1. Small Molecules 3475 12.2. Biomolecules 3477 12.3. Materials 3477 13. List of Abbreviations 3477 14. Acknowledgments 3478 15. References 3478

Tools and tactics for the optical detection of mercuric ion.

Synthesis of Highly Fluorescent Gold Nanoparticles for Sensing Mercury(II)

This study describes the first instance of the use of two differently sized Au nanoparticles (Au NPs), acting separately as donor and acceptor, in homogeneous photoluminescence quenching assays developed for the analysis of proteins. Introduction of a breast cancer marker protein, platelet-derived growth factor AA (PDGF AA), to a solution of 11-mercaptoundecanoic acid-protected, 2.0-nm photoluminescent Au nanodots (LAuND) led to the preparation of PDGF AA−LAuND as the donor. Thiol-derivative PDGF binding aptamers (Apt) and 13-nm spherical Au NPs were used to synthesize the Apt−QAuNP acceptor. The photoluminescence of PDGF AA−LAuND at 520 nm decreased when photoluminescence quenching occurred between Apt−QAuNP and PDGF AA−LAuND. We used the PDGF AA−LAuND/Apt−QAuNP-based molecular light switching system to analyze PDGFs and PDGF α-receptor in separate homogeneous solutions. In the presence of PDGFs, the interaction between Apt−QAuNP and PDGF AA−LAuND decreased as a result of competitive reactions between th...

Bioconjugated Gold Nanodots and Nanoparticles for Protein Assays Based on Photoluminescence Quenching

Colorimetric determination of urinary adenosine using aptamer-modified gold nanoparticles.

We describe the use of titanium dioxide nanoparticles (TiO2 NPs) as selective probes and matrices for the determination of catechins using surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS). The interactions between the enediol compounds and TiO2 NPs were evident by the change in color of the TiO2 NP solution from milky white to orange. Through these interactions, the TiO2 NPs could be used to concentrate enediol compounds, including catechins and ascorbic acid. The limits of detection (LODs) for three catechins – catechin, (−)-epigallocatechin, and (−)-epigallocatechin gallate – at a signal-to-noise ratio of 3 were 0.45, 1.85 and 0.65 µM, respectively. The TiO2 NP matrices provide a number of advantages over conventional organic matrices (e.g. 2′,4′,6′-trihydroxyacetophenone), including ease of sample preparation, less background noise in the low-mass region, and high repeatability. The applicability of this method was confirmed through the high reproducibility of the determination of the two catechins in tea samples that had not been subjected to any sample preparation procedures (shot-to-shot variation: <10%). Copyright © 2007 John Wiley & Sons, Ltd.

Determining enediol compounds in tea using surface‐assisted laser desorption/ionization mass spectrometry with titanium dioxide nanoparticle matrices

We have investigated the interactions between a variety of thiols and Nile Red-adsorbed gold nanoparticles (NRAuNPs) After adding thiols to solutions of NRAuNPs, the solutions fluoresce strongly as a result of the displacement of a Nile Red-derived product from the surface of the AuNPs We propose a mechanism for the formation of this NR product on the surface of AuNPs by conducting mass spectrometry, fluorescence, and capillary electrophoresis measurements By recording the fluorescence changes of the NRAuNP solutions after addition of the thiols, we investigated the interactions between the thiols and NRAuNPs Using the Langmuir isotherm model, we found that the displacement rate constants for thiols having one carboxyl residue, such as 3-mercaptopropionic acid, fall within the range 055 – 119 × 10 −2 s −1  Thiols containing hydroxyl groups [eg, 2-mercaptoethanol (2-ME)] or amino groups [eg, N-(2-mercaptopropionyl)glycine (MPG)], or that have flat structures on the AuNP surface, such as mercaptosuccinic acid, exhibit double-exponential kinetics with first rate constants of 051 – 283 × 10 −2 s −1 and second rate constants of 60 – 234 × 10 −4 s −1  Our results reveal that steric effects and the charge density of the thiols both play important roles in determining the interactions with NRAuNPs The interactions (displacement and/or induced aggregation) are also dependent on the size of NRAuNPs

Kun-Hong Lee

Papers

Synthesis of Highly Fluorescent Gold Nanoparticles for Sensing Mercury(II)

Bioconjugated Gold Nanodots and Nanoparticles for Protein Assays Based on Photoluminescence Quenching

Colorimetric determination of urinary adenosine using aptamer-modified gold nanoparticles.

Determining enediol compounds in tea using surface‐assisted laser desorption/ionization mass spectrometry with titanium dioxide nanoparticle matrices

Fluorescence and interactions with thiol compounds of Nile Red-adsorbed gold nanoparticles.