Colorimetric Assay for S-Adenosylhomocysteine Hydrolase Activity and Inhibition Using Fluorosurfactant-Capped Gold Nanoparticles

Gold nanocluster-based fluorescent sensors for in vitro and in vivo ratiometric imaging of biomolecules

Gold nanoclusters (AuNCs) are promising nanomaterials for ratiometric fluorescent probes due to their tunable fluorescence wavelengths dependent on size and structure, as well as their biocompatibility and resistance to photobleaching. By incorporating an additional fluorescence spectral peak, dual-emission AuNC-based fluorescent probes have been developed to enhance the signal output reproducibility. These probes can be fabricated by integrating various luminescent nanomaterials with AuNCs. This review focuses on the preparation methods and applications of ratiometric fluorescent probes derived from AuNCs and other fluorescent nanomaterials or fluorescent dyes for both in vitro and in vivo bioimaging of target analytes. Additionally, the review delves into the sensing mechanisms of AuNC-based ratiometric probes, their synthetic strategies, and the challenges encountered when using AuNCs for ratiometric bioimaging. Moreover, we explore the application of protein-stabilized AuNCs and thiolate-capped AuNC-based ratiometric fluorescent probes for biosensing and bioimaging. Two primary methods for assembling AuNCs and fluorophores into ratiometric fluorescent probes are discussed: triggered assembly and self-assembly. Finally, we address the challenges and issues associated with ratiometric bioimaging using AuNCs and propose future directions for further advancing AuNCs as ratiometric imaging agents.

Photothermal immunoassay for carcinoembryonic antigen based on the inhibition of cysteine-induced aggregation of gold nanoparticles by copper ion using a common thermometer as readout

The dispersed gold nanoparticles (AuNPs) have weak photothermal effect in near-infrared (NIR) region. After the addition of cysteine, the AuNPs are aggregated due to the electrostatic interaction and then exhibited strong photothermal effect. At present of copper ion (Cu²⁺), the cysteine was catalytically oxidized into cystine, leading to the inhibition of the aggregation of AuNPs and the photothermal effect decreased. Based on this, a simple photothermal assay can be developed for Cu²⁺ detection using a common thermometer as readout. The change of the temperature (ΔT) of the system has a linear relationship with Cu²⁺ in the range of 10-300 nM with a detection limit of 7.4 nM (S/N = 3). Furthermore, through labeling the detection antibody in immunoassay with CuO nanoparticles as the source of Cu²⁺, a convenient photothermal immunoassay can be developed. Carcinoembryonic antigen (CEA), an important biomarker for cancer screening, was chosen as the model target because the rise of CEA level is widely present in cancer blood serum. Under the optimized conditions, ΔT has a linear relationship with CEA concentration in the range of 3.0-48.0 ng/mL. The detection limit is 1.3 ng/mL. The proposed method had been applied to detect CEA in serum samples with good agreement with the reference method used in hospital.

Gold nanoparticle-based colorimetric biosensors

Gold nanoparticles (AuNPs) provide excellent platforms for the development of colorimetric biosensors as they can be easily functionalised, displaying different colours depending on their size, shape and state of aggregation. In the last decade, a variety of biosensors have been developed to exploit the extent of colour changes as nano-particles (NPs) either aggregate or disperse, in the presence of analytes. Of critical importance to the design of these methods is that the behaviour of the systems has to be reproducible and predictable. Much has been accomplished in understanding the interactions between a variety of substrates and AuNPs, and how these interactions can be harnessed as colorimetric reporters in biosensors. However, despite these developments, only a few biosensors have been used in practice for the detection of analytes in biological samples. The transition from proof of concept to market biosensors requires extensive long-term reliability and shelf life testing, and modification of protocols and design features to make them safe and easy to use by the population at large. Developments in the next decade will see the adoption of user friendly biosensors for point-of-care and medical diagnosis as innovations are brought to improve the analytical performances and usability of the current designs. This review discusses the mechanisms, strategies, recent advances and perspectives for the use of AuNPs as colorimetric biosensors.

A coupled photometric assay for characterization of S-adenosyl-l-homocysteine hydrolases in the physiological hydrolytic direction

S-Adenosyl-l-homocysteine hydrolases (SAHases) are important metabolic enzymes and their dysregulation is associated with some severe diseases. In vivo they catalyze the hydrolysis of S-adenosyl-l-homocysteine (SAH), the by-product of methylation reactions in various organisms. SAH is a potent inhibitor of methyltransferases, thus its removal from the equilibrium is an important requirement for methylation reactions. SAH hydrolysis is also the first step in the cellular regeneration process of the methyl donor S-adenosyl-l-methionine (SAM). However, in vitro the equilibrium lies towards the synthetic direction. To enable characterization of SAHases in the physiologically relevant direction, we have developed a coupled photometric assay that shifts the equilibrium towards hydrolysis by removing the product adenosine, using a high affinity adenosine kinase (AK). This converts adenosine to AMP and thereby forms equimolar amounts of ADP, which is phosphorylated by a pyruvate kinase (PK), in turn releasing pyruvate. The readout of the assay is the consumption of NADH during the lactate dehydrogenase (LDH) catalyzed reduction of pyruvate to lactic acid. The applicability of the assay is showcased for the determination of the kinetic constants of an SAHase from Bradyrhizobium elkanii (K_M,SAH 41±5μM, v_max,SAH 25±1μM/min with 0.13mg/mL enzyme). This assay is a valuable tool for in vitro characterization of SAHases with biotechnological potential, and for monitoring SAHase activity in diagnostics.

Rational Design of a Profluorescent Substrate for S-adenosylhomocysteine Hydrolase and its Applications in Bioimaging and Inhibitor Screening

S-Adenosylhomocysteine hydrolase (SAHase) is a cellular enzyme that plays a key role in the methylation process, and a potential drug target in the discovery of antiviral and anticancer agents. There is increasing interest in determining its activity in the biological and clinical fields with chemosensors but with limited success so far. Herein, we designed and developed for the first time an off/on-type of fluorogenic substrate (NADE) that is directly responsive to SAHase activity. NADE used 1,8-naphthalimide as the signal reporter and adenosine (Ade) as the reaction center; removal of the Ade moiety enhanced the fluorescence by >10-fold. Kinetic study showed that NADE followed a non-Michaelis-Menten pattern that corresponded to the allosteric behavior of SAHase. NADE showed excellent selectivity and functioned efficiently in cells, allowing the microscopic imaging of SAHase activity. NADE can also be used to identify and measure the effectiveness of inhibitors in a markedly superior way. In a word, NADE would be broadly useful in clinical applications and academic studies.

An enzyme-free colorimetric assay using hybridization chain reaction amplification and split aptamers

A novel unmodified gold nanoparticle (AuNP)-based colorimetric assay was demonstrated using split aptamers and the hybridization chain reaction (HCR) amplification strategy. Here, the aptamer was divided into a structure-switching hairpin probe (DNA probe H1 (or H1')) and a single-stranded probe (DNA probe H2 (or H2')). In the presence of the target, DNA probe H1 (or H1') could specifically capture the target with the assistance of DNA probe H2 (or H2') to form a stable complex. Subsequently, the hairpin structure of DNA probe H1 (or H1') was changed, and then a chain reaction of hybridization events between two other hairpin probes (H3 and H4) propagated, resulting in the formation of nicked double-helices. Since it was difficult for such nicked double-helices to inhibit salt-induced AuNP aggregation, a red-to-blue color change was observed. With the elegant amplification effect of HCR, this assay showed a low detection limit (15 nM for Hg(2+) and 1 μM for adenosine), which was lower than or at least comparable to previous AuNP-based methods. The novel strategy not only eliminated the requirements of enzymatic reactions, separation processes, chemical modifications, and sophisticated instruments, but also could be used for other targets only by simply changing the DNA probe sequences.

1,4-Benzenediboronic-Acid-Induced Aggregation of Gold Nanoparticles: Application to Hydrogen Peroxide Detection and Biotin-Avidin-Mediated Immunoassay with Naked-Eye Detection

Hydrogen-peroxide (H2O2)-induced growth of small-sized gold nanoparticles (AuNPs) is often implemented for H2O2 sensing and plasmonic immunoassay. In contrast, there is little-to-no information in the literature regarding the application of H2O2-inhibited aggregation of citrate-capped AuNPs. This study discloses that benzene-1,4-diboronic acid (BDBA) was effective in driving the aggregation of citrate-capped AuNPs through an interaction between α-hydroxycarboxylate of citrate and boronic acids of BDBA. The H2O2-mediated oxidation of BDBA resulted in the conversion of boronic acid groups to phenol groups. The oxidized BDBA was incapable of triggering the aggregation of citrate-capped AuNPs. Thus, the presence of H2O2 prohibited BDBA-induced aggregation of citrate-capped AuNPs. The BDBA-induced aggregation of citrate-capped AuNPs can be paired with the glucose oxidase (GOx)-glucose system to design a colorimetric probe for glucose. Moreover, a H2O2·BDBA·AuNP probe was integrated with sandwich immunoassay, biotinylated antibody, and avidin-conjugated GOx for the selective naked-eye detection of rabbit immunoglobulin G (IgG) and human-prostate-specific antigen (PSA). The lowest detectable concentrations of rabbit IgG and human PSA by the naked eye were down to 0.1 and 4 ng/mL, respectively. More importantly, the proposed plasmonic immunoassay allowed the naked-eye quantification of 0-10 ng/mL PSA at an interval of 2 ng/mL in plasma samples.

Gold nanoparticles as sensitive optical probes

Recent advances in Au NP based optical sensing systems for various analytes based on absorption, fluorescence and SERS are summarized.

Tween 20-stabilized gold nanoparticles combined with adenosine triphosphate-BODIPY conjugates for the fluorescence detection of adenosine with more than 1000-fold selectivity

This study describes the development of a simple, enzyme-free, label-free, sensitive, and selective system for detecting adenosine based on the use of Tween 20-stabilized gold nanoparticles (Tween 20-AuNPs) as an efficient fluorescence quencher for boron dipyrromethene-conjugated adenosine 5'-triphosphate (BODIPY-ATP) and as a recognition element for adenosine. BODIPY-ATP can interact with Tween 20-AuNPs through the coordination between the adenine group of BODIPY-ATP and Au atoms on the NP surface, thereby causing the fluorescence quenching of BODIPY-ATP through the nanometal surface energy transfer (NSET) effect. When adenosine attaches to the NP surface, the attached adenosine exhibits additional electrostatic attraction to BODIPY-ATP. As a result, the presence of adenosine enhances the efficiency of AuNPs in fluorescence quenching of BODIPY-ATP. The AuNP-induced fluorescence quenching of BODIPY-ATP progressively increased with an increase in the concentration of adenosine; the detection limit at a signal-to-noise ratio of 3 for adenosine was determined to be 60nM. The selectivity of the proposed system was more than 1000-fold for adenosine over any adenosine analogs and other nucleotides. The proposed system combined with a phenylboronic acid-containing column was successfully applied to the determination of adenosine in urine.

Combined tween 20-stabilized gold nanoparticles and reduced graphite oxide-Fe3O4 nanoparticle composites for rapid and efficient removal of mercury species from a complex matrix

This study describes a simple method for removing mercuric ions (Hg(2+)) from a high-salt matrix based on the use of Tween-20-stabilized gold nanoparticles (Tween 20-Au NPs) as Hg(2+) adsorbents and composites of reduced graphite oxide and Fe3O4 NPs as NP collectors. Citrate ions adsorbed on the surface of the Tween 20-Au NPs reduced Hg(2+) to Hg(0), resulting in the deposition of Hg(0) on the surface of the NPs. To circumvent time-consuming centrifugation and transfer steps, the Hg(0)-containing gold NPs were collected using reduced graphite oxide-Fe3O4 NP composites. Compared with the reported NP-based methods for removing Hg(2+), Tween 20-Au NPs offered the rapid (within 30 min), efficient (>99% elimination efficiency), durable (>10 cycles), and selective removal of Hg(2+), CH3Hg(+), and C2H5Hg(+) in a high-salt matrix without the interference of other metal ions. This was attributed to the fact that the dispersed Tween 20-Au NPs exhibited large surface-area-to-volume ratio to bind Hg(2+) through Hg(2+)-Au(+) metallophilic interactions in a high-salt matrix. The formation of graphite oxide sheets and reduced graphite oxide-Fe3O4 NP composites was demonstrated using X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectrometry, and transmission electron microscopy. The mechanism of interaction between Tween 20-Au NPs and Hg(2+) was studied using visible spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy.

Cationic surfactant-based colorimetric detection of Plasmodium lactate dehydrogenase, a biomarker for malaria, using the specific DNA aptamer

A simple, sensitive, and selective colorimetric biosensor for the detection of the malarial biomarkers Plasmodium vivax lactate dehydrogenase (PvLDH) and Plasmodium falciparum LDH (PfLDH) was demonstrated using the pL1 aptamer as the recognition element and gold nanoparticles (AuNPs) as probes. The proposed method is based on the aggregation of AuNPs using hexadecyltrimethylammonium bromide (CTAB). The AuNPs exhibited a sensitive color change from red to blue, which could be seen directly with the naked eye and was monitored using UV-visible absorption spectroscopy and transmission electron microscopy (TEM). The reaction conditions were optimized to obtain the maximum color intensity. PvLDH and PfLDH were discernible with a detection limit of 1.25 pM and 2.94 pM, respectively. The applicability of the proposed biosensor was also examined in commercially available human serum.

Design of two and three input molecular logic gates using non-Watson–Crick base pairing-based molecular beacons

A single, resettable, and sensitive molecular beacon has been developed to operate two-input, three-input, and set–reset logic gates.

Visual detection of hexokinase activity and inhibition with positively charged gold nanoparticles as colorimetric probes

Positively charged gold nanoparticles can effectively differentiate ATP and ADP, thus providing a simple and visual approach to colorimetric detection of hexokinase activity and inhibition.

Determination of cysteine, homocysteine, cystine, and homocystine in biological fluids by HPLC using fluorosurfactant-capped gold nanoparticles as postcolumn colorimetric reagents

We have demonstrated for the first time the suitability of fluorosurfactant-capped spherical gold nanoparticles as HPLC postcolumn colorimetric reagents for the direct assay of cysteine, homocysteine, cystine, and homocystine. The success of this work was based on the use of an on-line tris(2-carboxyethyl)phosphine reduction column for cystine and homocystine. Several parameters affecting the separation efficiency and the postcolumn colorimetric detection were thoroughly investigated. Under the optimized conditions, cysteine, homocysteine, cystine, and homocystine in human urine and plasma samples were determined. Detection limits for cysteine, homocysteine, cystine, and homocystine ranged from 0.16-0.49 μM. The accuracy in terms of recoveries ranged between 94.0-102.1%. This proposed method was rapid, inexpensive, and simple.

Highly selective colorimetric detection of spermine in biosamples on basis of the non-crosslinking aggregation of ssDNA-capped gold nanoparticles

The selective adsorption of single-stranded oligonucleotides (ssDNA) on gold nanoparticles (AuNPs) is well known for stabilizing the AuNPs against aggregation even at high salt concentrations. Our investigation shows that the non-crosslinking aggregation of arbitrary ssDNA-capped AuNPs occurs due to their interaction with the cationic polyamine, spermine (Spm), even without any addition of NaCl. The non-crosslinking aggregation mechanism is that the Spm, served as multivalent counterions, plays the dual roles of charge shielding and ion bridging among the ssDNA-capped AuNPs, which jointly result in the aggregation of the ssDNA-capped AuNPs. Therefore, a sensitive and highly selective colorimetric method for the detection of Spm was developed. To the best of our knowledge, it is the first successful case as to the efforts towards the development of optical assays for cationic polyamine, showing neither natural UV absorption nor fluorescence. Compared with the traditional methods of chromatography and capillary electrophoresis, the approach described here would provide a convenient alternative and new train of thought for the specific detection of Spm in both biological fluid and fermented products.

Improving colorimetric assays through protein enzyme-assisted gold nanoparticle amplification

The discovery of the DNA-mediated assembly of gold nanoparticles was a great moment in the history of science; this understanding and chemical control enabled the rational design of functional nanomaterials as novel probes in biodetection. In contrast with conventional probes such as organic dyes, gold nanoparticles exhibit high photostability and unique size-dependent optical properties. Because of their high extinction coefficients and strong distance dependent optical properties, these nanoparticles have emerged over the past decade as a promising platform for rapid, highly sensitive colorimetric assays that allow for the visual detection of low concentrations of metal ions, small molecules, and biomacromolecules. These discoveries have deepened our knowledge of biological phenomena and facilitated the development of many new diagnostic and therapeutic tools. Despite these many advances and continued research efforts, current nanoparticle-based colorimetric detection systems still suffer from several drawbacks, such as limited sensitivity and selectivity. This Account describes the recent development of colorimetric assays based on protein enzyme-assisted gold nanoparticle amplification. The benefits of such detection systems include significantly improved detection sensitivity and selectivity. First, we discuss the general design of enzyme-modified nanoparticle systems in colorimetric assays. We show that a quantitative understanding of the unique properties of different enzymes is paramount for effective biological assays. We then examine the assays for nucleic acid detection based on different types of enzymes, including endonucleases, ligases, and polymerases. For each of these assays, we identify the underlying principles that contribute to the enhanced detection capability of nanoparticle systems and illustrate them with selected examples. Furthermore, we demonstrate that the combination of gold nanoparticles and specific enzymes can probe enzyme dynamics and function with high specificity, offering substantial advantages in both sensitivity and specificity over conventional detection methods. The screening of nuclease, methyltransferase, protease, and kinase activities can be colorimetrically performed in a straightforward manner. Finally, we discuss examples of colorimetric assays for metal ions and small molecules that constitute important advances toward visual monitoring of enzyme catalytic functions and gene expression. Although these enzyme-assisted assay methods hold great promise for myriad applications in biomedicine and bioimaging, the application of the described techniques in vivo faces formidable challenges. In addition, researchers do not fully understand the interactions of gold nanoparticles with enzyme molecules. This understanding will require the development of new techniques to probe enzyme substrate dynamics at the particle interface with higher spatial resolution and chemical specificity.

One-step conjugation chemistry of DNA with highly scattered silver nanoparticles for sandwich detection of DNA

DNA-silver nanoparticle (AgNP) conjugates were facilely prepared through a one-step method, and then used for the quantitative detection of HIV DNA with a sandwich strategy based on their strong plasmon resonance scattering signals.

Luciferase-based assay for adenosine: application to S-adenosyl-L-homocysteine hydrolase

S-Adenosyl-L-homocysteine hydrolase (SAHH) catalyzes the reversible conversion of S-adenosyl-L-homocysteine (SAH) to adenosine (ADO) and L-homocysteine, promoting methyltransferase activity by relief of SAH inhibition. SAH catabolism is linked to S-adenosylmethionine metabolism, and the development of SAHH inhibitors is of interest for new therapeutics with anticancer or cholesterol-lowering effects. We have developed a continuous enzymatic assay for adenosine that facilitates high-throughput analysis of SAHH. This luciferase-based assay is 4000-fold more sensitive than former detection methods and is well suited for continuous monitoring of ADO formation in a 96-well-plate format. The high-affinity adenosine kinase from Anopheles gambiae efficiently converts adenosine to adenosine monophosphate (AMP) in the presence of guanosine triphosphate. AMP is converted to adenosine triphosphate and coupled to firefly luciferase. With this procedure, kinetic parameters (K(m), k(cat)) for SAHH were obtained, in good agreement with literature values. Assay characteristics include sustained light output combined with ultrasensitive detection (10(-7) unit of SAHH). The assay is documented with the characterization of slow-onset inhibition for inhibitors of the hydrolase. Application of this assay may facilitate the development of SAHH inhibitors and provide an ultrasensitive detection for the formation of adenosine from other biological reactions.