A cooperative effect of bifunctionalized nanoparticles on recognition: sensing alkali ions by crown and carboxylate moieties in aqueous media

Mixed-Ligand gold nanoparticles based optical sensor array for the recognition and quantification of seven toxic metals

Sensor arrays use pattern recognition for the identification and quantification of analytes. In the presented work, a gold nanoparticle (GNP) based optical sensor array was employed to classify and quantify seven toxic metals (arsenic, barium, cadmium, cerium, chromium, lead, and mercury). The sensor array receptors were GNPs functionalized by mercaptoundecanoic acid, 2-mercaptoethanesulfonate, and a 1:1 mixture of the two ligands. The mixed-ligand particle responds to the same analytes as the mono-ligand particles but in a distinctive way. This behavior demonstrates the high potential of mixed-ligand particles in the fabrication of sensor array receptors. The responses of the GNPs to different concentrations of the seven metal ions were analyzed, and a unique "classification trajectory" was produced for every metal. Samples of different metal concentrations were then measured and identified using the "classification trajectories". Once sample composition has been identified, a PLSR model, produced from the concatenated sensor array spectra of four calibration samples for each nanoparticle, was used to determine the metal concentration.

DNA-assisted assembly of cationic gold nanoparticles: Monte Carlo simulation

DNA-assisted assembly of ligand-stabilized gold nanoparticles is studied using Monte Carlo simulations with coarse-grained models for DNA and AuNP. Their interaction in a periodic simulation box is described by a combination of electrostatic and pairwise hard core potentials. We first probe the self-assembly of AuNPs resulting in an ordered distribution on a single fixed DNA strand. Subsequently, the effective force calculated between a pair of parallel DNA in the presence of AuNPs shows the attraction between them at short distance associated to a stable equilibrium position. Finally, the osmotic pressure calculated in a compact DNA-AuNP lattice with various amounts of monovalent salt ions shows that an increasing amount of salt prevents aggregate formation.

Preparation, Functionalization, Modification, and Applications of Nanostructured Gold: A Critical Review

Gold nanoparticles (Au NPs) play a significant role in science and technology because of their unique size, shape, properties and broad range of potential applications. This review focuses on the various approaches employed for the synthesis, modification and functionalization of nanostructured Au. The potential catalytic applications and their enhancement upon modification of Au nanostructures have also been discussed in detail. The present analysis also offers brief summaries of the major Au nanomaterials synthetic procedures, such as hydrothermal, solvothermal, sol-gel, direct oxidation, chemical vapor deposition, sonochemical deposition, electrochemical deposition, microwave and laser pyrolysis. Among the various strategies used for improving the catalytic performance of nanostructured Au, the modification and functionalization of nanostructured Au produced better results. Therefore, various synthesis, modification and functionalization methods employed for better catalytic outcomes of nanostructured Au have been summarized in this review.

Metallic-Nanoparticle-Based Sensing: Utilization of Mixed-Ligand Monolayers

The last two decades have seen great advancements in fundamental understanding and applications of metallic nanoparticles stabilized by mixed-ligand monolayers. Identifying and controlling the organization of multiple ligands in the nanoparticle monolayer has been studied, and its effect on particle properties has been examined. Mixed-ligand protected particles have shown advantages over monoligand protected particles in fields such as catalysis, self-assembly, imaging, and drug delivery. In this Review, the use of mixed-ligand monolayer protected nanoparticles for sensing applications will be examined. This is the first time this subject is examined as a whole. Mixed-ligand nanoparticle-based sensors are revealed to be divided into four groups, each of which will be discussed. The first group consists of ligands that work cooperatively to improve the sensors' properties. In the second group, multiple ligands are utilized for sensing multiple analytes. The third group combines ligands used for analyte recognition and signal production. In the final group, a sensitive, but unstable, functional ligand is combined with a stabilizing ligand. The Review will conclude by discussing future challenges and potential research directions for this promising subject.

Functionalization of Gold Nanoparticles by Inorganic Entities

The great affinity of gold surface for numerous electron-donating groups has largely contributed to the rapid development of functionalized gold nanoparticles (Au-NPs). In the last years, a new subclass of nanocomposite has emerged, based on the association of inorganic molecular entities (IME) with Au-NPs. This highly extended and diversified subclass was promoted by the synergy between the intrinsic properties of the shell and the gold core. This review-divided into four main parts-focuses on an introductory section of the basic notions related to the stabilization of gold nanoparticles and defines in a second part the key role played by the functionalizing agent. Then, we present a wide range of inorganic molecular entities used to prepare these nanocomposites (NCs). In particular, we focus on four different types of inorganic systems, their topologies, and their current applications. Finally, the most recent applications are described before an overview of this new emerging field of research.

Study of the Stability of Functionalized Gold Nanoparticles for the Colorimetric Detection of Dipeptidyl Peptidase IV

In this report, we investigated three stabilization strategies of gold nanoparticles and their practical application for the visual detection of dipeptidyl peptidase IV (DPP-IV). Citrate-capped gold nanoparticles (Au NPs) are generally unstable in high-ionic-strength samples. Au NPs are easily tagged with various proteins and biomolecules rich in amino acids, leading to important biomedical applications including targeted drug delivery, cellular imaging, and biosensing. The investigated assays were based on different modes of stabilization, such as the incorporation of polyethylene glycol (PEG) groups, stabilizer peptide, and bifunctionalization. Although all approaches provided highly stable Au NP platforms demonstrated by zeta potential measurements and resistance to aggregation in a high-ionic-strength saline solution, we found that the Au NPs modified with a separate stabilizer ligand provided the highest stability and was the only platform that demonstrated sensitivity to the addition of DPP-IV, whilst PEGylated and peptide-stabilized Au NPs showed no significant response.

Silicon Quantum Dot-Based Fluorescence Turn-On Metal Ion Sensors in Live Cells

Multiple sensor systems are designed by varying aza-crown ether moiety in silicon quantum dots (SiQDs) for detecting individual Mg(2+), Ca(2+), and Mn(2+) metal ions with significant selectivity and sensitivity. The detection limit of Mg(2+), Ca(2+), and Mn(2+) can reach 1.81, 3.15, and 0.47 μM, respectively. Upon excitation of the SiQDs which are coordinated with aza-crown ethers, the photoinduced electron transfer (PET) takes place from aza-crown ether moiety to the valence band of SiQDs core such that the reduced probability of electron-hole recombination may diminish the subsequent fluorescence. The fluorescence suppression caused by such PET effect will be relieved after selective metal ion is added. The charge-electron binding force between the metal ion and aza-crown ether hinders the PET and thereby restores the fluorescence of SiQDs. The design of sensor system is based on the fluorescence "turn-on" of SiQDs while in search of the appropriate metal ion. For practical application, the sensing capabilities of metal ions in the live cells are performed and the confocal image results reveal their promising applicability as an effective and nontoxic metal ion sensor.

Colorimetric detection based on localised surface plasmon resonance of gold nanoparticles: Merits, inherent shortcomings and future prospects

Localised surface plasmon resonance (LSPR) of gold nanoparticles (AuNPs) has been exploited for two decades in analytical science and has proven to be a powerful tool for the detection of various kinds of substances including small molecules, ions, macro biomolecules and microbes. Detection can be performed by visual colour change observations, photometry or resonance light scattering. A wide range of applications have been studied in the areas of environmental, pharmaceutical and biological analysis and clinical diagnosis. In this article, some fundamental aspects and important applications involving LSPR of AuNPs are reviewed. Several inherent shortcomings of these techniques and possible strategies to circumvent them are discussed.

Layered Metal Nanoparticle Structures on Electrodes for Sensing, Switchable Controlled Uptake/Release, and Photo-electrochemical Applications

Layered metal nanoparticle (NP) assemblies provide highly porous and conductive composites of unique electrical and optical (plasmonic) properties. Two methods to construct layered metal NP matrices are described, and these include the layer-by-layer deposition of NPs, or the electropolymerization of monolayer-functionalized NPs, specifically thioaniline-modified metal NPs. The layered NP composites are used as sensing matrices through the use of electrochemistry or surface plasmon resonance (SPR) as transduction signals. The crosslinking of the metal NP composites with molecular receptors, or the imprinting of molecular recognition sites into the electropolymerized NP matrices lead to selective and chiroselective sensing interfaces. Furthermore, the electrosynthesis of redox-active, imprinted, bis-aniline bridged Au NP composites yields electrochemically triggered "sponges" for the switchable uptake and release of electron-acceptor substrates, and results in conductive surfaces of electrochemically controlled wettability. Also, photosensitizer-relay-crosslinked Au NP composites, or electrochemically polymerized layered semiconductor quantum dot/metal NP matrices on electrodes, are demonstrated as functional nanostructures for photoelectrochemical applications.

Differentiation of biothiols from other sulfur-containing biomolecules using iodide-capped gold nanoparticles

We describe here a simple method based on the aggregation of iodide-capped gold nanoparticles (GNPs) to differentiate biothiols from other sulfur-containing biomolecules.

Portable Nanoparticle-Based Sensors for Food Safety Assessment

The use of nanotechnology-derived products in the development of sensors and analytical measurement methodologies has increased significantly over the past decade. Nano-based sensing approaches include the use of nanoparticles (NPs) and nanostructures to enhance sensitivity and selectivity, design new detection schemes, improve sample preparation and increase portability. This review summarizes recent advancements in the design and development of NP-based sensors for assessing food safety. The most common types of NPs used to fabricate sensors for detection of food contaminants are discussed. Selected examples of NP-based detection schemes with colorimetric and electrochemical detection are provided with focus on sensors for the detection of chemical and biological contaminants including pesticides, heavy metals, bacterial pathogens and natural toxins. Current trends in the development of low-cost portable NP-based technology for rapid assessment of food safety as well as challenges for practical implementation and future research directions are discussed.

Synthesis of thiol derivatives of azobenzocrown ethers. The preliminary studies on recognition of alkali metal ions by gold nanoparticles functionalized with azobenzocrown and lipoic acid

The article presents the synthesis of novel 13- and 16-membered azobenzocrown derivatives with peripheral thiol moieties and preliminary studies assessing their possible application in plasmonic sensors based on gold nanoparticles. The effect of the length of the chain connecting the macrocycle with the thiol group and the effect of the presence of the additional functional compound, i.e. lipoic acid, on the sensor response was analyzed. Colloidal gold nanoparticles modified with a 16-membered crown with a thiol group on oxyethylene (compound 12) or oxybutylene (compound 13) linker was found to have good properties, allowing for detection of potassium ions in aqueous solutions at concentrations 8-20 mM for bifunctionalized nanogold and 4-26 mM for less stable, colloidal gold modified only with thiol derivatives of azobenzocrowns. The response towards potassium cations of bifunctionalized nanogold modified with compound 13 was more stable in time than for the system incorporating compound 12. Compound 13, obtained with the highest yield among all presented thiol derivatives of azobenzocrowns, was selected for further, more detailed, studies.

Highly selective and sensitive recognition of cobalt(II) ions directly in aqueous solution using carboxyl-functionalized CdS quantum dots as a naked eye colorimetric probe: applications to environmental analysis

Quantum dots (QDs) are usually used as fluorescent probe, and they are difficult to use in colorimetric detection. However, in this report carboxyl-functionalized CdS (COF-CdS) QDs were synthesized in aqueous solution for colorimetric detection following a classic method. On the basis of inducing the aggregation of COF-CdS QDs, a simple naked eye colorimetric method with high sensitivity and selectivity was developed for the sensing of Co(2+) ions in aqueous solutions. The Co(2+) ions induced COF-CdS QDs results in a marked enhancement of the UV-vis absorption spectra at 360 nm, and the process was accompanied by a visible color change from colorless to yellowish brown within 5 min, which proves a sensitive detection of Co(2+) ions. The sensing of Co(2+) ions can therefore be easily achieved by a UV-vis spectrophotometer or even by the naked eye. Under the optimized circumstances, this method yields excellent sensitivity (LOD = 0.23 μg mL(-1)) and selectivity toward Co(2+) ions. The calibration plot of (A - A(0)) at 360 nm against concentration of Co(2+) ions was linear over the range from 0.5 to 14 μg mL(-1) with a correlation coefficient of 0.9996. The accuracy and reliability of the method were further ascertained by recovery studies via standard addition method with percent recoveries in the range of 99.63-102.46%. The plausible mechanism for the color change reaction has also been discussed. Our attempt may provide a cost-effective, rapid, and simple solution for the inspection of Co(2+) ions in the presence of a complex matrix from environmental aqueous samples.

Orthogonally bifunctionalised polyacrylamide nanoparticles: a support for the assembly of multifunctional nanodevices

Polyacrylamide nanoparticles bearing two orthogonal reactive functionalities were prepared by reverse microemulsion polymerisation. Water-soluble photosensitisers and peptide or carbohydrate moieties were sequentially attached to the new nanospecies by orthogonal conjugations based on copper-catalysed azide-alkyne cycloaddition and isothiocyanate chemistry.

Crown ether assembly of gold nanoparticles: melamine sensor

Melamine toxicity causing the renal failure and death of animals and humans has recently attracted worldwide attention. Developing an easy, fast, and sensitive method for the routine melamine detection is of great importance. Herein, we report the colorimetric sensing of melamine, based on the 18-crown-6 ether functionalized gold nanoparticles (GNPs) through the formation of cavity complexes with amines. Based on high extinction coefficients and spectral sensitivity of the surface plasmon resonance band of the GNPs, the rapid and sensitive melamine detection was achieved both visually and spectroscopically. Under the optimal conditions, melamine could be selectively detected in a concentration range from 10 to 500 ppb with a limit of detection as 6 ppb (3σ), which is much lower than the strictest melamine safety requirement of 1 ppm. To demonstrate the selectivity and practicality of the method, melamine detection was realized in the real complex samples (dairy) with excellent analyte concentration recovery, indicating its applicability for real-time monitoring of toxins in common products. Crown ether assembly of GNP also opens a new route for the formation of three-dimensional pseudorotaxane-like assemblies of nanoparticles that can be applicable to a variety of amine-bearing ligands.

Inhibitory effect of target binding on hairpin aptamer sticky-end pairing-induced gold nanoparticle assembly for light-up colorimetric protein assay

Gold nanoparticles (GNPs) possessing strong distance-dependent optical properties and high extinction coefficients have emerged as important colorimetric materials. Almost all colorimetric studies are based on two working mechanisms: sandwich cross-linking and non-cross-linking systems. In the present study, a new working mechanism, hairpin sticky-end pairing-induced GNP assembly, is introduced based on the discovery of unique aggregation behavior of aptamer-functionalized GNPs. The salt-induced aggregation of oligonucleotide probe-modified GNPs can readily occur due to the sticky-end pairing effect while addition of target molecules favors the formation of the hairpin structure of probe sequences and substantially inhibits the nanoparticle assembly. Along this line, we developed a proof-of-concept colorimetric homogeneous assay using immunoglobulin E (IgE) as an analyte model via transforming a commonly designed "light-down" colorimetric biosensor into a "light-up" one. From the point of view of both conformational transition of aptamer and steric bulk, oligonucleotide-GNPs display an additional stability upon binding to target molecules. The assay showed an extremely high sensitivity from both naked eye observations and absorbance measurements. Compared with almost all existing IgE sensing strategies, the proposed colorimetric system possesses a substantially improved analytical performance. Investigating the assembly behavior of hairpin aptamer-modified GNPs could offer new insight into the dependence of the GNP properties on the structure switching and open a new way to design signaling probes and develop colorimetric assay schemes.

Ultrasensitive copper(II) detection using plasmon-enhanced and photo-brightened luminescence of CdSe quantum dots

Here, we present a simple platform for the use of the enhanced emission of 16-mercaptohexadecanoic acid (16-MHA) capped CdSe quantum dots (QDs) as a probe for ultrasensitive copper(II) detection. In this study, the photoluminescence (PL) of the QDs was first enhanced by Ag nanoprisms which were self-assembled on Si surfaces and then further increased by photobrightening. Using this approach, the control and different analytes could be readily probed all on a single platform using fluorescence microscopy. The enhanced PL intensity of CdSe QDs was selectively quenched in the presence of Cu(2+), accompanied by the emergence of a new red-shifted luminescence band. The quenching mechanism was found to be due to a cation exchange mechanism as confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Herein, we have demonstrated that this simple methodology can offer a rapid and reliable detection of Cu(2+) with a detection limit as low as 5 nM and a dynamic range up to 100 muM in a fixed fast reaction time of 5 min. The potential applications of this technique were tested in two ways, for mixed-ion solutions and in physiological fluids, and both experiments exhibited good selectivity toward Cu(2+).

Rapid and selective lead (II) colorimetric sensor based on azacrown ether-functionalized gold nanoparticles

A gold nanoparticle (AuNPs)-based simple and fast colorimetric sensor for selective detecting of Pb(II) in aqueous solution has been developed. Monodisperse AuNPs (approx. 2.0 nm diameter) has been prepared facilely and further modified with an alkanethiol-bearing monoazacrown ether terminus. These AuNPs are shown to selectively sense Pb(2+) through color change, which is visually discernible by an appearance of the surface plasmon band (SPB) at 520 nm. The recognition mechanism is attributed to the unique structure of the monoazacrown ether attached to AuNPs and metal sandwich coordination between two azacrown ether moieties that are attached to separate nanoparticles. This inter-particle cross-linking results in an aggregation and apparent color change from brown to purple. Additionally, TEM experiments support the optical absorption data proving the aggregation between azacrown ether-capped gold nanoparticles. This AuNP-based colorimetric assay is a facile and robust method and allows fast detection of Pb(2+) at ambient temperatures. More importantly, the developed technique does not utilize enzymatic reactions, light-sensitive dye molecules, lengthy protocols or sophisticated instrumentation.

Colorimetric assay of glutathione based on the spontaneous disassembly of aggregated gold nanocomposites conjugated with water-soluble polymer

This article describes the glutathione-triggered disassembly of gold nanocomposites composed of gold cores and water-soluble copolymers [poly(N-n-isopropylacrylamide-co-acryloyldiethyletriamine)] attached to the surfaces of gold cores. The gold nanocomposites exhibit a bluish purple color because of the assembled gold cores that are conjugated with the diethylenetriamine groups incorporated into the copolymers. Glutathione added to the gold nanocomposite solution adsorbs onto the surface of the gold cores to liberate diethylenetriamine groups, resulting in spontaneous disassembly that changes the color of the solution to a reddish shade. Increasing the glutathione concentration facilitates the spontaneous disassembly of the gold nanocomposites. For the determination of glutathione, the colorimetric change of the gold nanoparticles is quantified with the a* value of the L*a*b* color coordinates defined by the CIE (Commission Internationale de l'Eclairage) chromaticity diagram. A linear relationship between the a* value and the glutathione concentration of up to 6 x 10(-6) mol/L is obtained 15 min after the addition of glutathione that has a detection limit (defined as 3sigma) of 2.9 x 10(-8) mol/L. The colorimetric assay is successfully applied to the determination of glutathione in eye drops and health supplements.