Optical detection of phenolic compounds based on the surface plasmon resonance band of Au nanoparticles

An indirect colorimetric method is presented for detection of trace amounts of hydroquinone (1), catechol (2) and pyrogallol (3). The reduction of AuCl4(-) to Gold nanoparticles (Au-NPs) by these phenolic compounds in the presence of cetyltrimethylammonium chloride (CTAC) produced very intense surface plasmon resonance peak of Au-NPs. The plasmon absorbance of Au-NPs allows the quantitative colorimetric detection of the phenolic compounds. The calibration curves derived from the changes in absorbance at lambda = 568 nm were linear with concentration of hydroquinone, catechol and pyrogallol in the range of 7.0 x 10(-7) to 1.0 x 10(-4)M, 6.0 x 10(-6) to 2.0 x 10(-4)M and 6.0 x 10(-7) to 1.0 x 10(-4)M, respectively. The detection limits were 5.3 x 10(-7), 2.5 x 10(-6) and 3.2 x 10(-7)M for the hydroquinone, catechol and pyrogallol, respectively. The method was applied satisfactorily to the determination of phenolic compounds in water samples and pharmaceutical formulations.     

Simple optical determination of silver ion in aqueous solutions using benzo crown-ether modified gold nanoparticles

We describe a method for the modification of gold nanoparticles (Au-NPs) with benzo-15-crown-5 that led to the development of a colorimetric assay for Ag(I) ion. The brown color of a solution of the modified Au-NPs turns to purple on addition of Ag(I) ion. The ratio of the UV–vis absorption at 600 nm and 525 nm is proportional to the concentration of Ag(I) ions in the range from 20 to 950 nM, and the detection limit is 12.5 nM. Other metal ions do not interfere if present in up to millimolar concentrations. The method enables a rapid determination of Ag(I) in lake and drinking water and is amenable to bare-eye readout.

Selective visual detection of Pb(II) ion via gold nanoparticles coated with a dithiocarbamate-modified 4′-aminobenzo-18-crown-6

We have developed a crown ether based selective colorimetric sensing scheme for the determination of Pb(II) ion by using gold nanoparticles modified with dithiocarbamate derivative of 4′-aminobenzo-18-crown-6 that acts as a colorimetric probe. Monodisperse Au-NPs were prepared by reacting 4′-aminobenzo-18-crown-6 with carbon disulfide to generate the dithiocarbamate ligand which was then added to the Au-NPs to form a supramolecular assembly on their surface. The Au-NPs modified in this way undergo aggregation in the presence of Pb(II) ions, and this causes the color to change from red to blue. The Pb(II)-induced aggregation can be monitored by using UV-visible spectrometry and even with the bare eye. The absorbance ratio (A_650nm/A_520nm) is linearly related to the concentration of Pb(II) in the 0.1 to 75 μM range (with a correlation coefficient of 0.9957), and the detection limit is 50 nM which is lower than the allowable level (75 nM) as defined by the US EPA. The method was successfully applied to the determination of Pb(II) in spiked water samples.

Schematic representation of Pb²⁺ ion-induced DTC-CE-Au NPs aggregation via sandwich complex formation.

     

Staining-free gel electrophoresis-based multiplex enzyme assay using DNA and peptide dual-functionalized gold nanoparticles

We report a simple staining-free gel electrophoresis method to simultaneously probe protease and nuclease. Utilizing gold nanoparticles (Au-NPs) dual-functionalized with DNA and peptide, the presence and concentration of nuclease and protease are determined concurrently from the relative position and intensity of the bands in the staining-free gel electrophoresis. The use of Au-NPs eliminates the need for staining processes and enables naked eye detection, while a mononucleotide-mediated approach facilitates the synthesis of DNA/peptide conjugated Au-NPs and simplifies the operation procedures. Multiplex detection and quantification of DNase I and trypsin are successfully demonstrated.     

Rapid and ultrasensitive colorimetric detection of mercury(II) by chemically initiated aggregation of gold nanoparticles

The article describes a method for rapid and visual determination of Hg(II) ion using unmodified gold nanoparticles (Au-NPs). It involves the addition of Au-NPs to a solution containing Hg(II) ions which, however, does not induce a color change. Next, a solution of lysine is added which induces the aggregation of the Au-NPs and causes the color of the solution to change from wine-red to purple. The whole on-site detection process can be executed in less than 15 min. Other amines (ethylenediamine, arginine, and melamine) were also investigated with respect to their capability to induce aggregation. Notably, only amines containing more than one amino group were found to be effective, but a 0.4 μM and pH 8 solution of lysine was found to give the best results. The detection limits for Hg (II) are 8.4 pM (for instrumental read-out) and 10 pM (for visual read-out). To the best of our knowledge, this LOD is better than those reported for any other existing rapid screening methods. The assay is not interfered by the presence of other common metal ions even if present in 1000-fold excess over Hg(II) concentration. It was successfully applied to the determination of Hg(II) in spiked tap water samples. We perceive that this method provides an excellent tool for rapid and ultrasensitive on-site determination of Hg(II) ions at low cost, with relative ease and minimal operation.

Rapid and ultrasensitive detection of mercury ions using gold nanoparticle based label-free colorimetric method with excellent sensitivity, easy operation and low cost.

     

Addition of gold nanoparticles to real-time PCR: effect on PCR profile and SYBR Green I fluorescence

Real-time quantitative polymerase chain reaction (qPCR) is the industry standard technique for the quantitative analysis of nucleic acids due to its unmatched sensitivity and specificity. Optimisation and improvements of this fundamental technique over the past decade have largely consisted of attempts to allow faster and more accurate ramping between critical temperatures by improving assay reagents and the thermal geometry of the PCR chamber. Small gold nanoparticles (Au-NPs) have been reported to improve PCR yield under fast cycling conditions. In this study, we investigated the effect of Au-NPs on optimised real-time qPCR assays by amplifying DNA sequences from genetically modified canola in the presence and absence of 0.9 nM Au-NPs of diameter 12 ± 2nm. Contrary to expectations, we found that Au-NPs altered the PCR amplification profile when using a SYBR Green I detection system due to fluorescence quenching; furthermore, high-resolution melt (HRM) analysis demonstrated that Au-NPs destabilised the double-stranded PCR product. The results indicate that effects on the assay detection system must be carefully evaluated before Au-NPs are included in any qPCR assay. Figure Raw amplification profiles in the presence and absence of gold nanoparticles     

Tryptophan-contained peptide-functional nanomaterials as general spectrofluorometric reagents for enzyme

By designing and coupling a functional peptide, Gly-Leu-Ala-Cys-Ser-Gly-Phe-Pro-Arg-Gly-Arg-Trp, which could be cleaved by thrombin at the site of Arg-Gly (R-G), to the surface of gold nanoparticles (Au-NPs), we propose a simple spectrofluorometry for thrombin (TRB) in this contribution. Experiments showed that the peptide coupled to the surface of Au-NPs in a Tris-HCl buffer at 37 degrees C could be cleaved, leaving the fluorescent fragment of Gly-Arg-Trp in the Au-NPs suspension. By centrifuging the suspension and measuring the fluorescence signals resulting from the Trp residue of Gly-Arg-Trp fragment in the supernatant, we found that the fluorescence intensity is proportional to thrombin concentrations in the range of 1-100 nM with the limit of the detection of 0.1 nM. Since there are a lot of enzymes that can hydrolyze peptide with special sequence, and novel nanomaterials that can bind with the tryptophan-contained peptide and understand centrifugation, this spectrofluorometric method is general and it is possible to develop a variety of detection method for target enzymes.     

Synthesis of 28-membered macrocyclic polyammonium cations functionalized gold nanoparticles and their potential for sensing nucleotides

A new synthesis of underivatized gold nanoparticles (Au-NPs) in water stabilized by the highly water soluble 28-membered macrocyclic polyammonium chloride, [28]ane-(NH(2)(+))(6)O(2)6Cl(-) (28-MCPAC) is reported. In addition to providing stability, 28-MCPAC with its cationic form functionalizes the Au-NPs for sensing anions in water. The 28-MCPAC-Au-NPs show a surface plasmon band in the visible region (>520 nm). By tuning the 28-MCPAC:HAuCl(4) ratio, Au-NPs with different core diameters ranging from 4 nm to 6 nm, as determined by TEM analysis, can be obtained. Particles are spherical, discrete, and appeared to have narrow size distributions. Raman spectroscopy confirms that the physisorption is responsible for the interaction between Au-NP surface and 28-MCPAC. The potential of the as-synthesized particles for sensing monophosphorylated nucleosides (nucleotides): 5-adenosine monophosphate (5-AMP), 5-cytosine monophosphate (5-CMP), 5-guanine monophosphate (5-GMP), and 5-uridine monophosphate (5-UMP) is investigated spectroscopically. Nucleotides-assisted agglomerations of 28-MCPAC-Au-NPs follow the order: 5-UMP>5-GMP>5-CMP>5-AMP. An attempt is taken to prepare Au-NPs in water at pH 4.55 without an added stabilizer. Particles without an added stabilizer are short lived, and the TEM image shows that the particles aggregate following a quasi-two-dimensional self-assembly array.     

Sensitive fluorescent detection of Staphylococcus aureus using nanogold linked CdTe nanocrystals as signal amplification labels

A sensitive fluorescent assay was developed for the detection of DNA specifically for Staphylococcus aureus. A sandwich-type detection system was fabricated by first immobilizing biotinylated capture DNA on avidin-modified wells of microplates, then hybridizing the capture DNA with one end of the target DNA, and then recognizing the other end of the target DNA with a signal probe labeled with CdTe nanocrystals and gold nanoparticles (Au-NPs) at the 3′- and 5′-terminus, respectively. Hybridization was monitored by measuring the fluorescent intensity of the assembly. The experimental results demonstrated that the incorporation of Au-NPs in this detection system can significantly enhance the sensitivity and the selectivity because a single Au-NP can be loaded with hundreds of signal DNA probe strands modified with CdTe nanocrystals. Under the optimized conditions, a detection limit of 10 fmol of DNA per L can be achieved and at least 50 colony forming units of Staph. aureus per mL of sample can be detected. The method was assessed by analyzing real samples, and it was validated by comparing it to an official standard method.

Colorimetric assay for lysozyme using Micrococcus luteus labeled with a blue dye, Remazol brilliant blue R, as a substrate.

Micrococcus luteus (M. lysodeikticus) labeled with Remazol brilliant blue R (blue ML) was prepared as a novel substrate for the colorimetric assay of lysozyme. The treatment of the labeled substrate with lysozyme resulted in the release of soluble blue products which can be easily measured spectrophotometrically at 600 nm. The blue color was most efficiently released at pH 7 and ionic strength of 0.2 on incubation with hen lysozyme at 40 degrees C. A new colorimetric method for the assay of lysozyme using this substrate was developed. The assay system gave a linear dose-response curve, and as little as 0.1 microgram of human lysozyme (1 microgram/ml, 100 microliters) can be detected. The present method is more convenient and reproducible than the conventional lysozyme assay with bacterial cells. Application of the system to the determination of lysozyme in human serum is described.     

A colorimetric method for highly sensitive and accurate detection of iodide by finding the critical color in a color change process using silver triangular nanoplates

In this contribution, we demonstrated a novel colorimetric method for highly sensitive and accurate detection of iodide using citrate-stabilized silver triangular nanoplates (silver TNPs). Very lower concentration of iodide can induce an appreciable color change of silver TNPs solution from blue to yellow by fusing of silver TNPs to nanoparticles, as confirmed by UV–vis absorption spectroscopy and transmission electron microscopy (TEM). The principle of this colorimetric assay is not an ordinary colorimetry, but a new colorimetric strategy by finding the critical color in a color change process. With this strategy, 0.1 μM of iodide can be recognized within 30 min by naked-eyes observation, and lower concentration of iodide down to 8.8 nM can be detected using a spectrophotometer. Furthermore, this high sensitive colorimetric assay has good accuracy, stability and reproducibility comparing with other ordinary colorimetry. We believe this new colorimetric method will open up a fresh insight of simple, rapid and reliable detection of iodide and can find its future application in the biochemical analysis or clinical diagnosis.     

Detection of flavonoids and assay for their antioxidant activity based on enlargement of gold nanoparticles

We report our findings that natural flavonoids such as quercetin, daizeol and puerarin can act as reductants for the enlargement of gold nanoparticles (Au-NPs). Consequently, the UV–vis spectra of a solution containing Au-NPs will be gradually changed, and the molecules of the natural herbs can be detected by making use of changes in the UV–visible spectra. Furthermore, we have prepared a self-assembled monolayer modified electrode by modifying cysteamine on a gold substrate electrode, which is further modified by some Au-NP seeds. When the modified electrode is immersed in a solution containing flavonoids and tetrachloroauric acid as a gold source for the growth of the Au-NP seeds, with the increase of the concentration of flavonoids, the Au-NP seeds on the surface of the modified electrode can be enlarged to varying degrees. As a result, the peak currents in the corresponding cyclic voltammograms are inversely decreased, and simultaneously the peak separation is increased. Therefore, an electrochemical method to detect flavonoids is also proposed. Compared with the optical detection method, the electrochemical method has an extraordinarily lower detection limit and a significantly extended detection range. Moreover, the optical and electrochemical experimental results can be also used to assay and compare the relative antioxidant activities of the flavonoids. Figure Enlargement of Au nanoparticles by flavonoids at cysteamine modified electrode     

A simple and rapid chemical method for the determination of cephalosporins

A simple and rapid chemical assay for cephalosporins is described. It is a simple modification of the colorimetric determination of penicillins in which the narrow spectrum beta-lactamase (penicillinase) is replaced by a broad spectrum beta-lactamase (cephalosporinase) produced by Enterobacter cloacae. The method can be used for assay of fermentation broths as well as for pure cephalosporins.     

Molecular junctions composed of oligothiophene dithiol-bridged gold nanoparticles exhibiting photoresponsive properties

Three oligothiophene dithiols with different numbers of thiophene rings (3, 6 or 9) have been synthesized and characterized. The X-ray single crystal structures of terthiophene 2 and sexithiophene 5 are reported herein to show the exact molecular lengths, and to explain the difference between their UV-visible spectra arising from the different packing modes. These dithiols with different chain lengths were then treated with 2-dodecanethiol-protected active gold nanoparticles (Au-NPs) by means of in situ thiol-to-thiol ligand exchange in the presence of 1 microm gap Au electrodes. Thus the molecular junctions composed of self-assembled films were prepared, in which oligothiophene dithiol-bridged Au-NPs were attached to two electrodes by means of Au-S bonded contacts. The morphologies and current-voltage (I-V) characteristics of these films were studied by SEM and AFM approaches, which suggested that the thickness of the films (3-4 layers) varied within the size of one isolated Au-NP and typical distance-dependent semiconductor properties could be observed. Temperature dependent I-V measurements for these molecular junctions were performed in which the films served as active elements in the temperature range 6-300 K; classical Arrhenius plots and subsequent linear fits were carried out to give the activation energies (deltaE) of devices. Furthermore, preliminary studies on the photoresponsive properties of these devices were explored at 80, 160, and 300 K, respectively. Physical and photochemical mechanisms were used to explain the possible photocurrent generation processes. To the best of our knowledge, this is the first report in which oligothiophene dithiols act as bridging units to link Au-NPs, and also the first report about functionalized Au-NPs exhibiting photoresponse properties in the solid state.     

Rapid colorimetric sensing of tetracycline antibiotics with in situ growth of gold nanoparticles

A colorimetric assay utilizing the formation of gold nanoparticles was developed to detect tetracycline antibiotics in fluidic samples. Tetracycline antibiotics showed the capability of directly reducing aurate salts into atomic gold which form gold nanoparticles spontaneously under proper conditions. The resulted gold nanoparticles showed characteristic plasmon absorbance at 526 nm, which can be visualized by naked eyes or with a spectrophotometer. UV–vis absorbance of the resulted gold nanoparticles is correlated directly with the concentrations of tetracycline antibiotics in the solution, allowing for quantitative colorimetric detection of tetracycline antibiotics. Reaction conditions, such as pH, temperature, reaction time, and ionic strength were optimized. Sensitivity of the colorimetric assay can be enhanced by the addition of gold nanoparticle seeds, a LOD as low as 20 ng mL⁻¹ can be achieved with the help of seed particles. The colorimetric assay showed minimum interference from ethanol, methanol, urea, glucose, and other antibiotics such as sulfonamides, amino glycosides etc. Validity of the method was also evaluated on urine samples spiked with tetracycline antibiotics. The method provides a broad spectrum detection method for rapid and sensitive detection of reductive substances such as tetracycline antibiotics in liquid and biological samples.     

Enzymatic coupling of specific peptides at nonspecific ligation sites: effect of Asp189Glu mutation in trypsin on substrate mimetic-mediated reactions

Two main drawbacks seriously restrict the synthetic value of proteases as reagents in peptide fragment coupling: (i) native proteolytic activity and, thus, risk of undesired peptide cleavage; (ii) limited enzyme specificities restricting the amino acid residues between which a peptide bond can be formed. While the latter can be overcome by the use of substrate mimetics achieving peptide bond formation at nonspecific ligation sites, the risk of proteolytic cleavage still remains and hinders the wide acceptance of this powerful strategy for peptide coupling. This paper reports on the effect of the trypsin point mutant Asp189Glu on substrate mimetic-mediated reactions. The effect of this mutation on the steady-state hydrolysis of substrate mimetics of the 4-guanidinophenyl ester type and on trypsin-specific Lys- and Arg-containing peptides was investigated. The results were confirmed by enzymatic coupling reactions using substrate mimetics as the acyl donor and specific amino acid-containing peptides as the acyl acceptor. The competition assay verifies the predicted shift in substrate preference from Lys and Arg to the substrate mimetics and, thus, from cleavage to synthesis of peptide bonds. The combination of results obtained qualifies the trypsin mutant D189E as the first substrate mimetic-specific peptide ligase.     

A colorimetric assay for the determination of polyhexamethylene biguanide in pool and spa water using nickel-nioxime

A novel nickel-nioxime analytical method to measure polyhexamethylene biguanide (PHMB) in swimming pools and spas was developed. This method utilizes nickel(II) chloride and 1,2-cyclohexanedionedioxime (nioxime) chemistry. In the method, nickel ions bind and neutralize PHMB in the solution. Excess, un-reacted nickel ions react with nioxime and the resulting colored solution is measured at 550 nm using a colorimetric assay. Currently, the colorimetric method to measure PHMB uses bromophenol blue (BPB). However, high levels of quaternary ammonium based algaecides and surfactant based products interfere with this colorimetric method. A time-consuming and expensive high-performance liquid chromatographic (HPLC) analysis can be used for samples with high levels of quaternary ammonium based algaecides or surfactants. The proposed nickel-nioxime detection method achieves comparable PHMB results to HPLC in about 5 min and is a very economical and simple method to perform.     

One-pot synthesis of robust core/shell gold nanoparticles

A one-pot synthesis of thermally stable core/shell gold nanoparticles (Au-NPs) was developed via surface-initiated atom transfer radical polymerization (ATRP) of n-butyl acrylate (BA) and a dimethacrylate-based cross-linker. The higher reactivity of the cross-linker enabled the formation of a thin cross-linked polymer shell around the surface of the Au-NP before the growth of linear polymer chains from the shell. The cross-linked polymer shell served as a robust protective layer, prevented the dissociation of linear polymer brushes from the surfaces of Au-NPs, and provided the Au-NPs excellent thermal stability at elevated temperature (e.g., 110 degrees C for 24 h). This synthetic method could be easily expanded for preparation of other types of inorganic/polymer nanocomposites with significantly improved stability.     

Selective optical sensing of biothiols with Ellman's reagent: 5,5′-Dithio-bis(2-nitrobenzoic acid)-modified gold nanoparticles

Development of sensitive and selective methods of determination for biothiols is important because of their significant roles in biological systems. We present a new optical sensor using Ellman's reagent (DTNB)-adsorbed gold nanoparticles (Au-NPs) (DTNB-Au-NP) in a colloidal solution devised to selectively determine biologically important thiols (biothiols) from biological samples and pharmaceuticals. 5,5′-Dithio-bis(2-nitrobenzoic acid) (DTNB), a versatile water-soluble compound for quantitating free sulfhydryl groups in solution, was adsorbed through non-covalent interaction onto Au-NPs, and the absorbance changes associated with the formation of the yellow-colored 5-thio-2-nitrobenzoate (TNB²⁻) anion as a result of reaction with biothiols was measured at 410 nm. The sensor gave a linear response over a wide concentration range of standard biothiols comprising cysteine, glutathione, homocysteine, cysteamine, dihydrolipoic acid and 1,4-dithioerythritol. The calibration curves of individual biothiols were constructed, and their molar absorptivities and linear concentration ranges determined. The cysteine equivalent thiol content (CETC) values of various biothiols using the DTNB-Au-NP assay were comparable to those of the conventional DTNB assay, showing that the immobilized DTNB reagent retained its reactivity toward thiols. Common biological sample ingredients like amino acids, flavonoids, vitamins, and plasma antioxidants did not interfere with the proposed sensing method. This assay was validated through linearity, additivity, precision and recovery, demonstrating that the assay is reliable and robust. DTNB-adsorbed Au-NPs probes provided higher sensitivity (i.e., lower detection limits) in biothiol determination than conventional DTNB reagent. Under optimized conditions, cysteine (Cys) was quantified by the proposed assay, with a detection limit (LOD) of 0.57 μM and acceptable linearity ranging from 0.4 to 29.0 μM (r = 0.998).     

Dual-signal immuno-competitive determination of brain natriuretic peptide based on magnetic nanozyme

Brain natriuretic peptide (BNP) has been a disease marker in the diagnosis of heart failure. In this study, gold nanoparticles modified with Hemin (H-AuNPs) as nanozymes were used to oxidize ABST and MB to amplified colorimetric and electrochemical redox signals respectively. BNP was combined with H-AuNPs (BNP-H-AuNPs) through electrostatic adsorption to construct competitive nanozyme probes. Target BNP in the sample compete with BNP-H-AuNPs to bind the antibody-modified magnetic nanoparticles (AntiBNP-MNPs). Due to the excellent catalytic performance of the nanozyme, BNP can be observed well by colorimetric and electrochemical assays. Electrochemical method ensured more accurate detection of BNP with a wide detection range (1–200 pg/mL) and a low detection of limit (0.03 pg/mL). Meanwhile, the results of the experiment can be easily observed with the naked eye by simple colorimetric method with a range from 5 ng/mL to 25 ng/mL and a limit of detection down to 80.3 pg/mL. Thus, based on the important role of H-AuNPs, this assay has exhibited potential value of detection the other small proteins through this competitive nanozyme method.