The authors describe a gold nanoparticle (AuNP) based aggregation assay for colorimetric determination of silver ions. The detection scheme is based on the release of aptamers from the surface of AuNPs that is triggered by the formation of C-Ag(I)-C links. In the absence of Ag(I) ions, the aptamers are readily adsorbed on the surface of the AuNPs. This prevents the aggregation of AuNPs and warrants the stability of the red colloidal solution at high salt concentration. In the presence of Ag(I) ions, the aptamers are released from the surface of AuNPs due to binding to Ag(I). Hence, salt-induced aggregation of AuNPs will occur which is accompanied by a gradual color change from red to blue. The color change occurs in the 1 to 500 nM Ag(I) concentration range, and the detection limit is 0.77 nM. The method was successfully applied to the determination of Ag(I) in spiked tap water samples.
Graphical abstract Schematic of a gold nanoparticle-based aggregation assay for colorimetric determination of silver ions. Visual quantitation also is posssible due to a gradual color change from red to blue.
A method is described for the determination of the polarity of mixed organic solvents by using the fluorescent probe Hostasol Red (HR) desposited on the outer surface of nanosized zeolite L. Organic solvents and their mixtures can be roughly classified according to their polarity with bare eyes and fluorometrically. Emission peaks range from 520 to 640 nm. Some solvents act as quenchers. The method is studied with series of protic and nonprotic solvents, and with selected mixtures of organic solvents.
Graphical abstract The dye Hostalene Red adsorbed on nanosized zeolite shows strong fluorescence solvatochromism. This can be exploited to quickly assess the polarity of solvents and solvent mixtures.
The negatively charged ruthenate(II) complex [Ru(bpy)(PPh3)(CN)3]? and gold nanoparticles (AuNPs) were used for detecting lysozyme (LYS). The luminescence of the ruthenate(II) complex is quenched by AuNPs, and this induces the aggregation of AuNPs and a color change from red to blue. After addition of lysozyme, the positively charged lysozyme and the negatively charged ruthenate(II) complex bind each other by electrostatic interaction firstly. This prevents AuNPs from aggregation and quenches the emission of the ruthenate(II) complex. Its luminescence and the degree of aggregation of the AuNPs can be used to quantify LYS. The fluorometric calibration plot is linear in the 0.01 to 0.20 μM LYS concentration range, and the calibration plot is linear between 0.02 and 0.20 μM of LYS. The color of the solution can be easily distinguished by bare eyes at 0.08 μM or higher concentration of LYS. The applicability of the method was verified by the correct analysis of LYS in chicken egg white.
Graphical abstract Schematic of a luminometric and colorimetric probe based on the induced aggregation of gold nanoparticles by an anionic luminescent ruthenate(II) complex or sensitive lysozyme detection.
This review (with (318) refs) describes progress made in the design and synthesis of morphologically different metal oxide nanoparticles made from iron, manganese, titanium, copper, zinc, zirconium, cobalt, nickel, tungsten, silver, and vanadium. It also covers respective composites and their function and application in the field of electrochemical and photoelectrochemical sensing of chemical and biochemical species. The proper incorporation of chemical functionalities into these nanomaterials warrants effective detection of target molecules including DNA hybridization and sensing of DNA or the formation of antigen/antibody complexes. Significant data are summarized in tables. The review concludes with a discussion or current challenge and future perspectives.
This review (with 85 refs.) summarizes the recent literature on the adsorption of common aromatic pollutants by using modified metal-organic frameworks (MOFs). Four kinds of aromatic pollutants are discussed, namely benzene homologues, polycyclic aromatic hydrocarbons (PAHs), organic dyes and their intermediates, and pharmaceuticals and personal care products (PPCPs). MOFs are shown to be excellent adsorbents that can be employed to both the elimination of pollutants and to their extraction and quantitation. Adsorption mechanisms and interactions between aromatic pollutants and MOFs are discussed. Finally, the actual challenges of existence and the perspective routes towards future improvements in the field are addressed.
Graphical abstract Recent advance on adsorption of common aromatic pollutants including benzene series, polycyclic aromatic hydrocarbons, organic dyes and their intermediates, pharmaceuticals and personal care products by metal-organic frameworks.
An aptamer based assay is described for the colorimetric detection of adenosine. The presence of adenosine triggers the deformation of hairpin DNA oligonucleotide (HP1) containing adenosine aptamer and then hybridizes another unlabeled hairpin DNA oligonucleotide (HP2). This leads to the formation of a double strand with a blunt 3′ terminal. After exonuclease III (Exo III)-assisted degradation, the guanine-rich strand (GRS) is released from HP2. Hence, the adenosine-HP1 complex is released to the solution where it can hybridize another HP2 and initiate many cycles of the digestion reaction with the assistance of Exo III. This leads to the generation of a large number of GRS strands after multiple cycles. The GRS stabilize the red AuNPs against aggregation in the presence of potassium ions. If, however, GRS forms a G-quadruplex, it loses its ability to protect gold nanoparticles (AuNPs) from salt-induced AuNP aggregation. Therefore, the color of the solution changes from red to blue which can be visually observed. This colorimetric assay has a 0.13 nM detection limit and a wide linear range that extends from 5 nM to 1 μM.
Graphical abstract Schematic presentation of a colorimetric aptamer biosensor for adenosine detection based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles.
A sensitive visual aptamer-based assay is presented for the determination of ractopamine (RAC) in animal feed beef. In the absence of RAC, the aptamer binds to gold nanoparticles (AuNPs) and this prevents the AuNPs to undergo salt-induced aggregation which usually is accompanied by a color change from red to blue. If however, RAC is present, it will bind to the aptamer while the AuNPs remain uncoated so that aggregation and a color change will occur due to salt-induced aggregation. This can be monitored by spectrophotometer or even with bare eyes. Under optimal conditions, the aptasensor exhibits a linear range that covers the 10 to 400 ng.mL￣1 RAC concentration range. The limit of detection is as low as 10 ng.mL￣1. In order to further improve selectivity, a RAC-selective molecularly imprinted membrane was prepared and used to pre-extract RAC from complex samples. The combined method (molecularly imprinted membrane and aptasensor) was applied to the determination of RAC in spiked animal feed and beef and gave recoveries that ranged from 72.7 % to 87.3 % for complete feed and from 78.2 % to 86.5 % for beef, respectively.
Graphical abstract A sensitive visual aptamer-based assay based on aggregation of gold nanoparticles in combination with a molecularly imprinted polymer was developed for the determination of ractopamine (RAC) in animal feed and beef.
This article reviews the progress made in the past 5 years in the field of direct and non-enzymatic electrochemical sensing of glucose. Following a brief discussion of the merits and limitations of enzymatic glucose sensors, we discuss the history of unraveling the mechanism of direct oxidation of glucose and theories of non-enzymatic electrocatalysis. We then review non-enzymatic glucose electrodes based on the use of the metals platinum, gold, nickel, copper, of alloys and bimetals, of carbon materials (including graphene and graphene-based composites), and of metal-metal oxides and layered double hydroxides. This review contains more than 200 refs.
Figure This article reviews the history of unraveling the mechanism of direct electrochemical glucose oxidation and the attempts to successfully develop non-enzymatic electrochemical glucose sensors over the past 5 years.
The authors describe four different kinds of sorbents for solid-phase extraction (SPE) and preconcentration of proteins from complex samples. All are based on the use of a poly(glycidyl-co-ethylene dimethacrylate) host monolith that was chemically functionalized by using two different ligands (ammonia and cysteamine). Gold nanoparticles (AuNPs) or silver NPs were then assembled to the amino or thiol groups. The resulting materials are shown to be viable stationary phases for use in SPE cartridges. The sorbents can selectively retain bovine serum albumin, and the thiol-modified sorbents containing AuNPs and AgNPs provide the highest recoveries (>90%) and satisfactory loading capacities (29.3 and 17.6 μg?mg?1 of sorbent, respectively). The applicability of these nanosorbents was demonstrated by preconcentrating viscotoxins from mistletoe extracts. The enriched fractions were subjected to MALDI-TOF analysis to underpin their selectivity.
Graphical abstract Hybrid materials based on methacrylate polymers modified with gold or silver nanoparticles were used as sorbents for solid phase extraction and preconcentration of bovine serum albumin and mistletoe viscotoxins, this followed by MALDI-TOF analysis.
The preparation and application of casein-capped gold nanoparticles (AuNPs) as a specific probe for ferric ions Fe(III) is reported. The functionalized AuNPs exhibit narrow size distribution and form stable dispersions in water of different ionic strengths and basicity. The presence of diverse functional groups from the side chain of peptides warrants colloidal stability of AuNPs and also assists recognition of Fe(III) in versatile conditions. Fe(III) ion reportedly causes the aggregation of AuNPs and a red-shift in absorbance toward longer wavelength (660 nm). A spectrophotometric method is appropriate for selective detection of Fe(III) and the spectral shift is also accompanied by a color change from red to blue. The aggregation of AuNPs is not suppressed after the addition of NaOH or at moderate ionic strength. The resulting spectrophotometric method works for Fe(III) in the concentration range of 0.1 to 0.9 μM and has a detection limit of 450 nM. The AuNP probe can also detect Fe(III) ion content in real samples at the same detection limit, which is much lower than the maximum contaminant level allowed for Fe(III) in drinking water (5.37 μM) by the U.S. Environmental Protection Agency.
Graphical abstract Casein peptide functionalized gold nanoparticles: synthesis, characterization, and their application to the visual detection of Fe(III).
A colorimetric method is presented for the determination of the antibiotic ofloxacin (OFL) in aqueous solution. It is based on the use of an aptamer and gold nanoparticles (AuNPs). In the absence of OFL, the AuNPs are wrapped by the aptamer and maintain dispersed even at the high NaCl concentrations. The solution with colloidally dispersed AuNPs remains red and has an absorption peak at 520 nm. In the presence of OFL, it will bind to the aptamer which is then released from the AuNPs. Hence, AuNPs will aggregate in the salt solution, and color gradually turns to blue, with a new absorption peak at 650 nm. This convenient and specific colorimetric assay for OFL has a linear response in the 20 to 400 nM OFL concentration range and a 3.4 nM detection limit. The method has a large application potential for OFL detection in environmental and biological samples.
Graphical abstract Schematic of a sensitive and simple colorimetric aptasensor for ofloxacin (OFL) detection in tap water and synthesic urine. The assay is based on the salt-induced aggregation of gold nanoparticles which results in a color change from red to purple.
A method is described for the colorimetric determination of mercury(II). In the absence of Hg(II), aminopropyltriethoxysilane (APTES) which is positively charged at pH 7 is electrostatically absorbed on the surface of gold nanoparticles (AuNPs). This neutralizes the negative charges of the AuNPs and leads to NP aggregation and a color change from red to blue-purple. However, in the presence of Hg(II), reduced Hg (formed through the reaction between Hg(II) and citrate on the AuNP surface) will replace the APTES on the AuNPs. Hence, the formation of aggregates is suppressed and the color of the solution does not change. The assay is performed by measuring the ratio of absorbances at 650 and 520 nm and can detect Hg(II) at nanomolar levels with a 10 nM limit of detection. The specific affinity between mercury and gold warrants the excellent selectivity for Hg(II) over other environmentally relevant metal ions.
Graphical Abstract Schematic of the method for determination of Hg2+ based on the gold amalgam-induced deaggregation of gold nanoparticles in the presence of APTES with the LOD of 10.1 nM.
A colorimetric method is described for the determination of Pt(II). It is based on the use of gold nanoparticles (AuNPs) which are known to aggregate in the presence of a cationic polymer such as poly(diallyldimethylammonium chloride) (PDDA). If, however, a mismatched aptamer (AA) electrostatically binds to PDDA, aggregation is prevented. Upon the addition of Pt(II), it will bind to the aptamer and induce the formation of a hairpin structure. Hence, interaction between aptamer and PDDA is suppressed and PDDA will induce the aggregation of the AuNPs. This is accompanied by a color change from red to blue. The effect can be observed with bare eyes and quantified by colorimetry via measurement of the ratio of absorbances at 610 nm and 520 nm. Response is linear in the 0.24–2 μM Pt(II) concentration range, and the detection limit is 58 nM. The assay is completed within 15 min and selective for Pt(II) even in the presence of other metal ions. It was successfully applied to the rapid determination of Pt(II) in spiked soil samples.
Graphical abstract Schematic representation of the method for detection of Pt(II) based on the use of a cationic polymer and gold nanoparticles. In the presence of Pt(II), aptamer interacts with the Pt(II) and prevents the interaction between aptamer and cationic polymer. Hence, cationic polymer induce the aggregation of the AuNPs and lead to the color change from red to blue.
A study is presented on the binding kinetics and mechanism of the adsorption of dsDNA on citrate-capped gold nanoparticles (AuNPs). Methods include fluorescence titration, isothermal calorimetry (ITC) titration, dynamic light scattering and gel electrophoresis. It is found that the fluorescence of probe DNA (labeled with Rhodamine Green and measured at excitation/emission peaks of 498/531 nm) is quenched by addition of AuNPs. The Stern-Volmer quenching constant (Ksv) is 1.67?×?10^9 L·mol?1 at 308 K and drops with increasing temperature. The quenching mechanism is mainly static. The results of both fluorescence titrations and ITC show negative values for ΔH and ΔS values. This shows ion-induced dipole-dipole interaction to be the main attractive forces between dsDNA and AuNPs, while electrostatic interactions result in repulsion. The repulsive forces lead to a lower affinity between dsDNA and AuNPs (compared to single-strand DNA). It is also found that dsDNA can prevent the aggregation of AuNPs which is accompanied by a color change from red into blue. The visual detection limit with bare eyes for dsDNA1 is 36 pM. Based on these findings, a colorimetric method was developed to detect the proto-oncogene of serine/threonine-protein kinase B-Raf V600E point mutation in HT29, Ec109, A549, Huh-7 and SW480 cell lines.
Graphical abstract Schematic of the salt-induced aggregation of uncapped gold nanoparticles (AuNPs) which leads to a color change from red to blue. If the AuNPs are coated with dsDNA, aggregation is suppressed.
The review (with 95 refs.) starts with an introduction that addresses the need for magnetic actuation in microfluidics. A second section describes the equations governing magnetic micromixing, with subsections on magnetic equations, fluid flow equations, and on convection–diffusion equations. The next section specifically covers magnetically actuated micromixers, with subsections on those actuated by external permanent magnets, by electromagnets, by microstirrers, and on micromixers with integrated electrodes. The conclusion summarizes the state of the art and addresses current challenges and trends.
Graphical abstract In this review, micromixers are classified into four types according to drive mode including external permanent magnet, electromagnet, microstirrer and the integrated electrode. The basic governing equations and operating rules of magnetic micromixers are given. The review is supposed to provide a helpful reference for those intending to study this field.
The authors report that the peroxidase-like activity of Au@Pt core-shell nanohybrids (Au@PtNHs) is selectively inhibited by cysteine. This finding has led to a highly sensitive colorimetric assay for cysteine that is based on the nanohybrid-catalyzed oxidation of TMB by H2O2 to form a blue product. The method has a detection limit of 5.0 nM and a linear range from 10 nM to 20 μM. The assay is highly selective over other amino acids. It was successfully applied to the determination of cysteine in an injection containing a mixture of amino acids.
Graphical abstract The peroxidase-like activity of Au@Pt core-shell nanohybrids (Au@PtNHs) is selectively inhibited by cysteine, enabling the determination of cysteine.
An efficient approach is demonstrated for preparing particles consisting of a silver core and a shell of molecularly imprinted polymer (Ag@MIP). The MIP is prepared by using bisphenol A (BPA) as the template and 4-vinylpyridine as the functional monomer. The Ag@MIP fulfills a dual function in that the silver core acts as a SERS substrate, while the MIP allows for selective recognition of BPA. The Ag@MIP is characterized by scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, thermogravimetric analysis and Raman spectroscopy. The Raman intensity of Ag@MIP is higher than that of bare silver microspheres. The detection limit for BPA is as low as 10?9 mol·L?1.
Graphical abstract Schematic illustration of the preparation of silver microspheres coated with a molecularly imprinted polymer (Ag@MIPs) for detecting bisphenol A (BPA) by surface enhanced Raman scattering (SERS).
Hetero-dimeric magnetic nanoparticles of the type Au-Fe3O4 have been synthesised from separately prepared, differently shaped (spheres and cubes), monodisperse nanoparticles. This synthesis was achieved by the following steps: (a) Mono-functionalising each type of nanoparticles with aldehyde functional groups through a solid support approach, where nanoparticle decorated silica nanoparticles were fabricated as an intermediate step; (b) Derivatising the functional faces with complementary functionalities (e.g. amines and carboxylic acids); (c) Dimerising the two types of particles via amide bond formation. The resulting hetero-dimers were characterised by high-resolution TEM, Fourier transform IR spectroscopy and other appropriate methods.
Graphical Abstract Nano-LEGO: Assembling two types of separately prepared nanoparticles into a hetero-dimer is the first step towards complex nano-architectures. This study shows a solid support approach to combine a gold and a magnetite nanocrystal.
The authors describe an aptamer-based detection scheme that is based on untemplated nucleic acid elongation and the use of copper nanoparticles (CuNPs) as a fluorescent probe. An aptamer without any other auxiliary sequence and label is required only which makes the method rather convenient. Under the catalysis of terminal deoxynucleotidyl transferase (TdT), the single-stranded aptamer is elongated without template. By using dTTPs as the substrate, long linear poly T can be produced, and these can act as templates for the synthesis of CuNPs which display red (617 nm) fluorescence under 349 nm photoexcitation. In the presence of the analyte, the TdT-catalyzed production of poly T is blocked, and this results in suppressed fluorescence. The strategy was successfully applied to the determination of the proteins thrombin and vascular endothelial growth factor 165. Only three steps are involved in the whole assay. This aptamer-based assay is believed to have a wide scope in that it may be applied to the analysis of many other proteins if the corresponding aptamers are available.
Graphical abstract A versatile aptamer-based method for the determination of thrombin and VEGF165 is introduced. It is based on TdT catalyzed nucleic acid elongation and poly T templated formation of fluorescent copper nanoparticles.
Conducting polymers possess good conductivity, can be easily modified, have a particular redox activity. Noble metal nanomaterials, in turn, possess high conductivity, catalytic properties and large surface-to-volume ratios. Synergistic materials consisting of both conducting polymer and metal nanomaterial therefore are most useful materials for use in electrochemical immunosensors with improved sensitivity and specificity. This review (with 75 references) gives an overview on advances in conducting polymer based noble metal nanomaterial hybrids for amperometric immunoassay of the 13 most common tumor markers. The review is divided into the following sections: (1) Polyaniline based noble metal nanomaterial hybrids; (2) Polyaniline derivative-based noble metal nanomaterial hybrids; (3) Polypyrrole-based noble metal nanomaterial hybrids. A final section covers future perspectives regarding challenges on the design of electrochemical immunoassays.
Graphical abstract Advances on conducting polymer and noble metal nanomaterial hybrids for amperometric immunoassay of tumor marker are reviewed. Future perspectives regarding challenges on the construction of electrochemical immunosensing interface for tumor marker are discussed.
