Colorimetric Sensitivity of Gold Nanoparticles: Minimizing Interparticular Repulsion as a General Approach

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.

A proof of concept application of aptachain: ligand-induced self-assembly of a DNA aptamer

A challenge for the use of aptamers as biosensors is how to signal the occurrence of their ligand binding event into a signal that can be exploited in a detection scheme. Here, we present the concept of “aptachain” formation, where an aptamer is split into two overlapping or staggered strands and assembles into an extended oligomer upon ligand binding. This assembly of aptamers can then be used as a way to detect ligand binding by the aptamer. As an example of this concept, we employed the cocaine-binding aptamer as a model system, used its ability to tightly bind quinine and demonstrated its capability in a gold nanoparticle-based biosensing application. We used isothermal titration calorimetry to demonstrate that, when split into two overlapping DNA strands, the aptamer remains functional. Size-exclusion chromatography showed that the quinine-bound oligos form a larger assembly of aptamer units than in the absence of ligand. Finally, we used the oligomer forming ability of the aptachain oligos in a biosensor application for quinine that brings gold nanoparticles closer together resulting in a shift in their plasmonic resonance to a longer wavelength and an observed colour shift. We propose that splitting aptamers into overlapping strands that form oligomers in the presence of a ligand, aptachain formation, will be generally applicable to aptamers and prove useful in a variety of biotechnology applications.

We present the concept of aptachain. An aptamer is split into two overlapping strands that form an oligomer when it binds its target. Aptachain formation can be used to detect ligand binding and may be beneficial in other biotechnology applications.

Zwitterionic peptide-capped gold nanoparticles for colorimetric detection of Ni2+

Zwitterionic nanoparticles are typically utilized as nanoprobes and delivery vehicles in nanomedicine and therapeutics due to their resistance to interferences. Their high stability also shows great potential to be applied in sensing applications. Here, we report a selective, sensitive and rapid colorimetric sensing of nickel ions (Ni²⁺) using zwitterionic polypeptide, EKEKEKPPPPC (EK)₃, capped gold nanoparticles (AuNP-(EK)₃). By taking advantage of the alternate carboxylic (-COOH)/amine (-NH₂) groups, the zwitterionic peptide can function dually by being able to sense metal ions and maintain colloidal stability. Ni²⁺ can trigger the aggregation of the AuNP-(EK)₃ nanoprobe, which results in a red-to-purple color change of the AuNP-(EK)₃ solution. Our 40 nm AuNP-(EK)₃ nanoprobe can detect Ni²⁺ as low as 34 nM within 15 min with a linear range of 60-160 nM, and is stable in soil, urine and water samples. We demonstrate that the aggregation mechanism of the nanoprobe is due to the interactions between the -NH₂ group of glutamic acid at the N-terminus of the peptide and Ni²⁺, and the aggregation process is reversible. Furthermore, the slight modification of two amino acid sequences at the N-terminus allows the nanoprobe to retain its stability, even in a high ionic strength medium. We believe that by adjusting or extending the peptide sequences, new metal ion selective peptides could be created.

Simple Colorimetric Detection of Amyloid β-peptide (1-40) based on Aggregation of Gold Nanoparticles in the Presence of Copper Ions

A simple method for specific colorimetric sensing of Alzheimer's disease related amyloid-β peptide (Aβ) is developed based on the aggregation of gold nanoparticles in the presence of copper ion. The detection of limit for Aβ(1-40) is 0.6 nM and the promising results from practical samples (human serum) indicate the great potential for the routine detection.

Sensitive and selective colorimetric sensing of Fe3+ion by using p-amino salicylic acid dithiocarbamate functionalized gold nanoparticles

DTC-PAS-Au NPs successfully acted as probes for the selective and sensitive colorimetric sensing of Fe³⁺ions in biological samples.

Gold nanoparticles in biomedical applications: recent advances and perspectives

Gold nanoparticles (GNPs) with controlled geometrical, optical, and surface chemical properties are the subject of intensive studies and applications in biology and medicine. To date, the ever increasing diversity of published examples has included genomics and biosensorics, immunoassays and clinical chemistry, photothermolysis of cancer cells and tumors, targeted delivery of drugs and antigens, and optical bioimaging of cells and tissues with state-of-the-art nanophotonic detection systems. This critical review is focused on the application of GNP conjugates to biomedical diagnostics and analytics, photothermal and photodynamic therapies, and delivery of target molecules. Distinct from other published reviews, we present a summary of the immunological properties of GNPs. For each of the above topics, the basic principles, recent advances, and current challenges are discussed (508 references).

Applications of nanotechnology in food packaging and food safety: barrier materials, antimicrobials and sensors

In this article, several applications of nanomaterials in food packaging and food safety are reviewed, including: polymer/clay nanocomposites as high barrier packaging materials, silver nanoparticles as potent antimicrobial agents, and nanosensors and nanomaterial-based assays for the detection of food-relevant analytes (gasses, small organic molecules and food-borne pathogens). In addition to covering the technical aspects of these topics, the current commercial status and understanding of health implications of these technologies are also discussed. These applications were chosen because they do not involve direct addition of nanoparticles to consumed foods, and thus are more likely to be marketed to the public in the short term.

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.

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.

Highly sensitive gold nanoparticle-based colorimetric sensing of mercury(II) through simple ligand exchange reaction in aqueous media

A strategy for the rational design of a novel colorimetric sensor based on dithioerythritol-modified gold nanoparticles for the selective recognition of Hg2+ in aqueous media is presented. This approach relies on the combination of gold nanoparticles with Hg2+ through sulfur-Hg2+-sulfur interaction. The gold nanoparticles showed high selectivity toward Hg2+ with binding-induced red shift in the absorption spectra, with no response to major interfering cations such as Pb2+, Cd2+, and Cu2+ in the presence of ethylenediamine tetraacetic acid. The system responds to Hg2+ with a detection limit of 100 nM and might open a new avenue for the development of Hg2+ sensing probes.

Effects of Mn2+ on oligonucleotide-gold nanoparticle hybrids for colorimetric sensing of Hg2+: improving colorimetric sensitivity and accelerating color change

In this paper, we present a simple and rapid colorimetric assay--using the polythymine oligonucleotide T(33), citrate-capped gold nanoparticles (AuNPs), and phosphate-buffer saline (PBS) in the presence of Mn(2+)--for the highly selective and sensitive detection of Hg(2+) in an aqueous solution. Citrate-capped AuNPs adsorbed on randomly coiled T(33) were dispersed well in PBS because of strong electrostatic repulsion between DNA molecules. In the presence of Hg(2+), the formation of Hg(2+)-T(33) complexes enabled the removal of T(33) molecules from the NP surface, resulting in salt-induced NP aggregation. However, the T(33)-capped AuNPs (T(33)-AuNPs) were dispersed in PBS solution after the addition of 1.0 microM Hg(2+), indicating that T(33)-AuNPs had poor colorimetric sensitivity toward Hg(2+). We uncovered that the addition of Mn(2+) to a solution containing 0.75 nM T(33)-AuNPs and 0.2x PBS resulted in an acceleration of the analysis time (within 5 min) and a 100-fold sensitivity improvement for the detection of Hg(2+). As a result, the present approach enables the analysis of Hg(2+) with a minimum detectable concentration that corresponds to 10 nM. This is probably attributed to that Mn(2+) binds strongly to the phosphate backbone of DNA, thereby accelerating Hg(2+)-induced aggregation of the T(33)-AuNPs. Because Mn(2+) can stabilize the folded structure of the Hg(2+)-T(33) complex, Hg(2+) facilitates the removal of T(33) from the NP surface in the presence of Mn(2+). This probe was successfully applied to the determination of Hg(2+) in pond water.