A colorimetric and surface-enhanced Raman scattering dual-signal sensor for Hg2+ based on Bismuthiol II-capped gold nanoparticles

A Critical Review on the Development of Optical Sensors for the Determination of Heavy Metals in Water Samples. The Case of Mercury(II) Ion

Recent publications are reviewed concerning the development of sensors for the determination of mercury in drinking water, based on spectroscopic methodologies. A critical analysis is made of the specific details and figures of merit of the developed protocols. Special emphasis is directed to the validation and applicability to real samples in the usual concentration range of mercury, considering the maximum allowed limits in drinking water established by international regulations. It was found that while most publications describe in detail the synthesis, structure, and physicochemical properties of the sensing phases, they do not follow the state of the art in the analytical developments. Recommendations are provided regarding the proper method development and validation, including the setting of the calibration concentration range, the correct estimation of the limits of detection and quantitation, the concentration levels to be set for producing spiked water samples, the number of real samples for adequate validation, the comparison of the developed method with a reference technique, and other analytical features which should be followed.

Microporous Oxide-Based Surface-Enhanced Raman Scattering Film for Quadrillionth Detection of Mercury Ion (II)

A variety of chemical sensing materials and procedures for conveniently detecting mercuric ion (II) (Hg2+) have been extensively explored. The detection challenges for accomplishing a simple, fast, and low investment procedure at the ultrasensitive level are ongoing. Herein we report a quadrillionth level for detecting Hg2+ by the surface-enhanced Raman scattering (SERS) technique. There is an interaction of silver nanoparticles decorated on a zinc-oxide tetrapod structure and coated on FTO glass (Ag@ZnO-FTO) with an organic ligand. 4,4′-Dipyridyl (DPy) performed as being chemisorbed by Ag nanoparticles interacting with a pyridine ring to produce plasmonic hot spots for SERS. The morphology of the surface and porous structure of the tetrapod becomes the powerful platform for enhanced SERS performance of DPy detection. In the absence of the augmentative electrolyte, the enhancement factor for DPy is more than 107. The inhibiting of the aggregation between Ag and DPy was present following the appearance of Hg2+, demonstrated by the quenching of the SERS signal from the DPy molecules. The capability to reproduce and the selectivity of the sensing by DPy were both demonstrated. In addition, the applications for detecting Hg2+ in natural water and beverages were successfully detected. These results demonstrated the SERS sensors had the potential for detecting Hg2+ in practical use.

A SERS-based capillary sensor for the detection of mercury ions in environmental water

Mercury ion (Hg²⁺) is one of the most toxic heavy metal ions which will cause permanent damage to the brain and kidneys. So, it is important to develop a sensitive, simple and reliable approach to detect Hg²⁺. In this work, we report a surface-enhanced Raman scatting (SERS) sensor by decorating the inner wall of capillary with 4,4'-dipyridyl (Dpy) functionalized silver nanoparticles (AgNPs). The main advantage of this sensor is that it can collect samples directly by capillary force and carry out on-site analysis by combining portable Raman spectrometer. In the presence of Hg²⁺, the Dpy molecules would be separated from the surface of AgNPs and coordinated with Hg²⁺, resulting in a decrease in the SERS signal. A linear correlation of Raman intensity with Hg²⁺ concentrations from 1 to 100 part-per-billion (ppb) was obtained for quantitative analysis and the limit of detection (LOD) was determined to be 0.1 ppb. The good reproducibility and selectivity of the sensor were also demonstrated. In addition, the sensors were successfully applied to detect Hg²⁺ in real environmental water samples, and the sampling process provided operation convenience compared to conventional methods. These results indicated that these capillary sensors had great potential for Hg²⁺ detection in practical use.

Simultaneous colorimetric and surface-enhanced Raman scattering detection of melamine from milk

We present a dual-mode readout sensing mechanism that can effectively distinguish true and false-positive signals of melamine in milk by combining colorimetric analysis and surface-enhanced Raman scattering (SERS) spectroscopy. The colorimetry analysis takes advantage of color change of plasmonic nanoparticles upon the presence of melamine. We discovered that Ag colloids with 20 nm diameter was suitable for both colorimetric and SERS methods. However, the colorimetric method may present false-positive signals with the presence of interfering compounds. SERS spectroscopy can overcome this limitation and directly obtain signature spectra from the same plasmonic NPs used for the colorimetric assay without any modification. Melamine/s-triazine can be reliably differentiated by probing the SERS spectra based on surface-selection rules. The limit of detection of sensing melamine from milk by this method could reached to 0.05 ppm. Therefore, the combination of colorimetric and SERS method not only allows for rapid preliminary screening of melamine by naked eyes, but also greatly reduces false-positive signals by surface selection rules in SERS.

Controlling the Morphologies of Silver Aggregates by Laser-Induced Synthesis for Optimal SERS Detection

Controlling the synthesis of metallic nanostructures for high quality surface-enhanced Raman scattering (SERS) materials has long been a central task of nanoscience and nanotechnology. In this work, silver aggregates with different surface morphologies were controllably synthesized on a glass-solution interface via a facile laser-induced reduction method. By correlating the surface morphologies with their SERS abilities, optimal parameters (laser power and irradiation time) for SERS aggregates synthesis were obtained. Importantly, the characteristics for largest near-field enhancement were identified, which are closely packed nanorice and flake structures with abundant surface roughness. These can generate numerous hot spots with huge enhancement in nanogaps and rough surface. These results provide an understanding of the correlation between morphologies and SERS performance, and could be helpful for developing optimal and applicable SERS materials.

A novel long-alkyl-chained acylhydrazone-based supramolecular polymer gel for the ultrasensitive detection and separation of multianalytes

By rationally introducing multi-self-assembly driving forces and coordination binding sites into the same molecule, a designed functional gelator, G, was synthesized. Next, a novel supramolecular polymer material, OGV (1% DMSO), was constructed and used for the ultrasensitive detection and separation of multianalytes in gel states. Interestingly, OGV showed a fluorescent ultrasensitive response for the Hg2+ and Fe3+ ions in water. Moreover, by introducing these metal ions into the OGV, stable metal ion-coordinated supramolecular metallogels (HgG and FeG) were formed, which could sense CN- and H2PO4- in water with high selectivity and sensitivity.

Thiosemicarbazone@Gold nanoparticle hybrid as selective SERS substrate for Hg2+ ions

The Raman spectral profile of p-methylcarbohydrazonethioamide (MCHT) is completely changed due to strong SERS effects upon bonding onto gold nanoparticles surface, but some vibrational modes are further enhanced in the presence of Hg(II) ions. The lack of SERS response for most common metal ions indicates that the coordinating groups are interacting with the gold nanoparticles surface and not available for binding metal ions in solution, except for mercury ions. The selective enhancement of some vibrational modes is consistent with significant conformational changes upon binding of Hg(II) ion onto the AuNP@MCHT hybrid, as confirmed by TEM/EDS measurements, demonstrating its potentiality as a highly selective and sensitive SERS substrate.

Determination of mercury (II) ions based on silver-nanoparticles-assisted growth of gold nanostructures: UV-Vis and surface enhanced Raman scattering approaches

Innovative dual detection methods for mercury(II) ions (Hg^(II)) have been developed based on the formation of gold nanostructures (AuNSs) following the addition of mercury-containing solution to a mixture containing an optimized amount of Au^(III), H₂O₂, HCl, and silver nanoparticles (AgNPs). In the absence of Hg^(II), the addition of Au^(III), H₂O₂, and HCl to the AgNP solution changes the solution's color from yellow to red, and the absorption peak shifts from 400 to 526nm, indicating the dissolution of AgNPs and the formation of gold nanoparticles (AuNPs). Because of the spontaneous redox reaction of Hg^(II) toward AgNPs, the change in the amount of remaining AgNP seed facilitates the generation of irregular AuNSs, resulting in changes in absorption intensity and shifting the peak within the range from 526 to 562nm depending on the concentration of Hg^(II). Under optimal conditions, the limit of detection (LOD) for Hg^(II) at a signal-to-noise ratio (S/N) of 3 was 0.3μM. We further observed that AgNP-assisted catalytic formation of Au nanomaterials deposited on a surface enhanced Raman scattering active substrate significantly reduced the Raman signal of 4-mercaptobenzoic acid, dependent on the Hg^(II) concentration. A linear relationship was observed in the range 0.1nM-100μM with a LOD of 0.05nM (S/N 3.0). As a simple, accurate and precise method, this SERS-based assay has demonstrated its success in determining levels of Hg^(II) in real water samples.

A novel supramolecular polymer gel based on naphthalimide functionalized-pillar[5]arene for the fluorescence detection of Hg2+ and I- and recyclable removal of Hg2+via cation-π interactions

The development of novel materials for the detection and removal of Hg²⁺ is a very important issue due to the acute toxicity of Hg²⁺. Herein, a novel supramolecular polymer P5BD-DPHB has been constructed by the collaboration of a naphthalimide functionalized-pillar[5]arene host (P5BD) and a bis-bromohexane functionalized-pillar[5]arene guest (DPHB). P5BD-DPHB could form a stable supramolecular gel (P5BD-DPHB-G). Interestingly, P5BD-DPHB-G shows selective fluorescent "turn-on" detection for Hg²⁺via cation-π interactions with high selectivity and sensitivity. Furthermore, the Hg²⁺ coordinated supramolecular gel P5BD-DPHB-HgG can detect I^- successively. The detection limits for Hg²⁺ and I^- are 1.65 × 10^-9 and 1.84 × 10^-8 mol L^-1, respectively. Even more significantly, the xerogel of P5BD-DPHB-G could remove Hg²⁺ from aqueous solution with excellent recyclability and ingestion capacity, and with a Hg²⁺ removal rate of 98%.

A novel water soluble chemosensor based on carboxyl functionalized NDI derivatives for selective detection and facile removal of mercury(ii)

A novel water-soluble Hg²⁺ sensor M2 has been designed and synthesized, which can provide a fluorescent “turn-on” response when it detects Hg²⁺. More meaningfully, the sensor M2 can remove Hg²⁺ from water effectively.

Rapid detection of mercury contamination in water by surface enhanced Raman spectroscopy

Mercury (Hg) is a potent neurotoxin in fish, wildlife, and humans.

A simple and selective colorimetric mercury (II) sensing system based on chitosan stabilized gold nanoparticles and 2,6-pyridinedicarboxylic acid

The development of simple and cost-effective methods for the detection and treatment of Hg²⁺ in the environment is an important area of research due to the serious health risk that Hg²⁺ poses to humans. Colorimetric sensing based on the induced aggregation of nanoparticles is of great interest since it offers a low cost, simple, and relatively rapid procedure, making it perfect for on-site analysis. Herein we report the development of a simple colorimetric sensor for the selective detection and estimation of mercury ions in water, based on chitosan stabilized gold nanoparticles (AuNPs) and 2,6-pyridinedicarboxylic acid (PDA). In the presence of Hg²⁺, PDA induces the aggregation of AuNPs, causing the solution to change colors varying from red to blue, depending on the concentration of Hg²⁺. The formation of aggregated AuNPs in the presence of Hg²⁺ was confirmed using transmission electron microscopy (TEM) and UV-Vis spectroscopy. The method exhibits linearity in the range of 300nM to 5μM and shows excellent selectivity towards Hg²⁺ among seventeen different metal ions and was successfully applied for the detection of Hg²⁺ in spiked river water samples. The developed technique is simple and superior to the existing techniques in that it allows detection of Hg²⁺ using the naked eye and simple and rapid colorimetric analysis, which eliminates the need for sophisticated instruments and sample preparation methods.

Facile fabrication of an electrochemical aptasensor based on magnetic electrode by using streptavidin modified magnetic beads for sensitive and specific detection of Hg(2.)

In this work, a novel electrochemical aptasensor was developed for sensitive and specific detection of Hg(2+) based on thymine-Hg(2+)-thymine (T-Hg(2+)-T) structure via application of thionine (Th) as indicator signal. For the fabrication of the aptasensor, streptavidin modified magnetic beads (Fe3O4-SA) was firmly immobilized onto the magnetic glassy carbon electrode (MGCE) benefited from its magnetic character. Then biotin labeled T-riched single stranded DNA (Bio-ssDNA) connected with Fe3O4-SA specifically and steadily because of the specific binding capacity between streptavidin and biotin. The stable structure of T-Hg(2+)-T formed in the present of Hg(2+) provided convenience for the intercalation of Th. The detection of Hg(2+) was achieved by recording the differential pulse voltammetry (DPV) signal of Th. Under optimal experimental conditions, the linear range of the fabricated electrochemical aptasensor was 1-200nmol/L, with a detection limit of 0.33nmol/L. Furthermore, the proposed aptasensor may find a potential application for the detection of Hg(2+) in real water sample analysis.

Label-free and dual-amplified electrochemical detection of Hg2+ based on self-assembled DNA nanostructures and target-triggered exonuclease cleavage activity

An electrochemical biosensor for Hg²⁺ detection via HCR and Hg²⁺-triggered Exo III-assisted target recycling for signal amplification.

Recent progress in detection of mercury using surface enhanced Raman spectroscopy--A review

Concerns over exposure to mercury have motivated the exploration of cost-effective, rapid, and reliable method for monitoring Hg(2+) in the environment. Recently, surface-enhanced Raman scattering (SERS) has become a promising alternative method for Hg(2+) analysis. SERS is a spectroscopic technique which combines modern laser spectroscopy with the optical properties of nano-sized noble metal structures, resulting in substantially increased Raman signals. When Hg(2+) is in a close contact with metallic nanostructures, the SERS effect provides unique structural information together with ultrasensitive detection limits. This review introduces the principles and contemporary approaches of SERS-based Hg(2+) detection. In addition, the perspective and challenges are briefly discussed.

Selective and Quantitative Detection of Trace Amounts of Mercury(II) Ion (Hg²⁺) and Copper(II) Ion (Cu²⁺) Using Surface-Enhanced Raman Scattering (SERS)

We report the development of a surface-enhanced Raman scattering (SERS)-based heavy metal ion sensor targeting the detection of mercury(II) ion (Hg(2+)) and copper(II) ion (Cu(2+)) with high sensitivity and selectivity. To achieve the detection of vibrational-spectroscopically silent heavy metal ions, the SERS substrate composed of gold nanorod (AuNR)-polycaprolactone (PCL) nanocomposite fibers was first functionalized using metal ion-binding ligands. Specifically, 2,5-dimercapto-1,3,4-thiadiazole dimer (di-DMT) and trimercaptotriazine (TMT) were attached to the SERS substrates serving as bridging molecules to capture Hg(2+) and Cu(2+), respectively, from solution. Upon heavy metal ion coordination, changes in the vibrational spectra of the bridging molecules, including variations in the peak-intensity ratios and peak shifts were observed and taken as indicators of the capture of the target ions. With rigorous spectral analysis, the coordination mechanism between the heavy metal ion and the corresponding bridging molecule was investigated. Mercury(II) ion primarily interacts with di-DMT through the cleavage of the disulfide bond, whereas Cu(2+) preferentially interacts with the heterocyclic N atoms in TMT. The specificity of the coordination chemistry provided both di-DMT and TMT with excellent selectivity for the detection of Hg(2+) and Cu(2+) in the presence of other interfering metal ion species. In addition, quantitative analysis of the concentration of the heavy metal ions was achieved through the construction of internal calibration curves using the peak-intensity ratios of 287/387 cm(-1) for Hg(2+) and 1234/973 cm(-1) for Cu(2+).

Detect, remove and reuse: a new paradigm in sensing and removal of Hg (II) from wastewater via SERS-active ZnO/Ag nanoarrays

Mercury being one of the most toxic heavy metals has long been a focus of concern due to its gravest threats to human health and environment. Although multiple methods have been developed to detect and/or remove dissolved mercury, many require complicated procedures and sophisticated equipment. Here, we describe a simple surface enhanced Raman spectroscopy (SERS) active ZnO/Ag nanoarrays that can detect Hg(2+), remove Hg(2+) and can be fully regenerated, not just from Hg(2+) contamination when heat-treated but also from the SERS marker when exposed to UV as a result of the self-cleaning ability of this schottky junction photocatalyst. The sensors are also highly selective because of the unique way mercury (among other chemicals) interacts with Ag nanoparticles, thus reducing its SERS activity.

Plasmon-enhanced optical sensors: a review

Surface plasmon resonance (SPR) has found extensive applications in chemi-sensors and biosensors. Plasmons play different roles in different types of optical sensors. SPR transduces a signal in a colorimetric sensor through shifts in the spectral position and intensity in response to external stimuli. SPR can also concentrate the incident electromagnetic field in a nanostructure, modulating fluorescence emission and enabling plasmon-enhanced fluorescence to be used for ultrasensitive detection. Furthermore, plasmons have been extensively used for amplifying a Raman signal in a surface-enhanced Raman scattering sensor. This paper presents a review of recent research progress in plasmon-enhanced optical sensing, giving emphasis on the physical basis of plasmon-enhanced sensors and how these principles guide the design of sensors. In particular, this paper discusses the design strategies for nanomaterials and nanostructures to plasmonically enhance optical sensing signals, also highlighting the applications of plasmon-enhanced optical sensors in healthcare, homeland security, food safety and environmental monitoring.

Synthesis of silver nanostructures by multistep methods

The shape of plasmonic nanostructures such as silver and gold is vital to their physical and chemical properties and potential applications. Recently, preparation of complex nanostructures with rich function by chemical multistep methods is the hotspot of research. In this review we introduce three typical multistep methods to prepare silver nanostructures with well-controlled shapes, including the double reductant method, etching technique and construction of core-shell nanostructures. The growth mechanism of double the reductant method is that different favorable facets of silver nanocrystals are produced in different reductants, which can be used to prepare complex nanostructures such as nanoflags with ultranarrow resonant band bandwidth or some silver nanostructures which are difficult to prepare using other methods. The etching technique can selectively remove nanoparticles to achieve the aim of shape control and is widely used for the synthesis of nanoflowers and hollow nanostructures. Construction of core-shell nanostructures is another tool to control shape and size. The three methods can not only prepare various silver nanostructures with well-controlled shapes, which exhibit unique optical properties, such as strong surface-enhanced Raman scattering (SERS) signal and localized surface plasmon resonance (LSPR) effect, but also have potential application in many areas.

An ultrasensitive label-free assay of 8-hydroxy-2'-deoxyguanosine based on the conformational switching of aptamer

We developed a highly sensitive label-free assay of 8-hydroxy-2'-deoxyguanosine (8-OHdG) using 8-OHdG-aptamer (Apt) as the recognition element. The Apt was adsorbed onto the surface of gold nanoparticles (AuNPs), which prevents them from aggregating under high-salt conditions. Upon addition of 8-OHdG, the conformation of the Apt changes to form a G-quadruplex structure, which leads to the aggregation of the AuNPs along with the increase of the resonance light scattering intensity. The mechanism of 8-OHdG that induces Apt to form G-quadruplexes structure was studied by circular dichroism. The response signals linearly correlated with the concentration of 8-OHdG ranging from 90.8pM to 14.1nM with a detection limit of 27.3pM, which is much lower than that obtained by other methods. This method does not need any label steps and sophisticated equipment. The application for detection of 8-OHdG in real samples further demonstrated its reliability. This strategy would be helpful for developing a universal analytical method by simply replacing corresponding aptamers for various target molecules.

Sensitive pseudobienzyme electrocatalytic DNA biosensor for mercury(II) ion by using the autonomously assembled hemin/G-quadruplex DNAzyme nanowires for signal amplification

Herein, a novel sensitive pseudobienzyme electrocatalytic DNA biosensor was proposed for mercury ion (Hg(2+)) detection by using autonomously assembled hemin/G-quadruplex DNAzyme nanowires for signal amplification. Thiol functionalized capture DNA was firstly immobilized on a nano-Au modified glass carbon electrode (GCE). In presence of Hg(2+), the specific coordination between Hg(2+) and T could result in the assembly of primer DNA on the electrode, which successfully triggered the HCR to form the hemin/G-quadruplex DNAzyme nanowires with substantial redox probe thionine (Thi). In the electrolyte of PBS containing NADH, the hemin/G-quadruplex nanowires firstly acted as an NADH oxidase to assist the concomitant formation of H2O2 in the presence of dissolved O2. Then, with the redox probe Thi as electron mediator, the hemin/G-quadruplex nanowires acted as an HRP-mimicking DNAzyme that quickly bioelectrocatalyzed the reduction of produced H2O2, which finally led to a dramatically amplified electrochemical signal. This method has demonstrated a high sensitivity of Hg(2+) detection with the dynamic concentration range spanning from 1.0 ng L(-1) to 10 mg L(-1) Hg(2+) and a detection limit of 0.5 ng L(-1) (2.5 pM) at the 3Sblank level, and it also demonstrated excellent selectivity against other interferential metal ions.

Detection of mercury ions based on mercury-induced switching of enzyme-like activity of platinum/gold nanoparticles

In this study, bimetallic platinum/gold nanoparticles (Pt/Au NPs) were found to exhibit peroxidase-like activity, and the deposition of mercury was found to switch the enzymatic activity to a catalase-like activity. Based on this phenomenon, we developed a new method for detecting mercury ions through their deposition on bimetallic Pt/Au NPs to switch the catalytic activity of Pt/Au NPs. Pt/Au NPs could be easily prepared through reduction of Au(3+) and Pt(4+) by sodium citrate in a one-pot synthesis. The peroxidase catalytic activity of the Pt/Au NPs was controlled by varying the ratios of Pt to Au. The Pt(0.1)/Au NPs (prepared with a [Au(3+)]/[Pt(4+)] molar ratio of 9.0/1.0) showed excellent oxidation catalysis for H(2)O(2)-mediated oxidation of Amplex® Red (AR) to resorufin. The oxidized product of AR, resorufin, fluoresces more strongly (excitation/emission wavelength maxima ca. 570/585 nm) than AR alone. The peroxidase catalytic activity of Pt(0.1)/Au NPs was switched to catalase-like activity in the presence of mercury ions in a 5.0 mM tris(hydroxymethyl)aminomethane (Tris)-borate solution (pH 7.0) through the deposition of Hg on the particle surfaces owing to the strong Hg-Au metallic bond. The catalytic activity of Hg-Pt(0.1)/Au NPs is superior (by at least 5-fold) to that of natural catalase (from bovine liver). Under optimal solution conditions [5.0 mM Tris-borate (pH 7.0), H(2)O(2) (50 mM), and AR (10 μM)] and in the presence of the masking agents polyacrylic acid and tellurium nanowires, the Pt(0.1)/Au NPs allowed the selective detection of inorganic mercury (Hg(2+)) and methylmercury ions (MeHg(+)) at concentrations as low as several nanomolar. This simple, fast, and cost-effective system enabled selective determination of the spiked concentrations of Hg(2+) and MeHg(+) in tap, pond, and stream waters.

Enhanced sensitivity of a direct SERS technique for Hg2+ detection based on the investigation of the interaction between silver nanoparticles and mercury ions

Mercury which is a very important pollutant has drawn significant attention in recent research. So far, among the various detection methods, the strategies based on surface-enhanced Raman scattering (SERS) are quite attractive because of the high sensitivity, and especially as it is reported that Hg(2+) can be directly detected by SERS without tagging. However, the procedure for the direct SERS detection of mercury is still unclear with little experimental evidence, limiting further development of Hg(2+) detection by SERS. Herein, we performed a simple method based on SERS for the detection of mercury ions in water without tagging. It is established that in only 2 min, low concentration of Hg(2+) can be recognized based on the decrease of SERS intensity. The detection procedure is investigated by multiple characterizations and the mechanism proven by the obtained data provides a practical way to further improve the sensitivity of the SERS detection. It is demonstrated that the interaction between Hg(2+) and Ag nanoparticles (Ag NPs) could occur in a short time, which includes the complexation of Hg(2+) with citrate and the formation of amalgam due to the reduction of Hg(2+). This interaction influences the surface plasmon resonance (SPR) property of Ag NPs and thereby decays the electromagnetic enhancement of Ag NPs; meanwhile the interaction also causes the zeta potential decrease of Ag NPs and accordingly affects the adsorption of Raman reporter molecules on the surface of Ag NPs. Therefore, the weakness of SERS intensity in the presence of Hg(2+) should be mainly attributed to the interaction between Hg(2+) and Ag NPs. From the mechanism demonstrated, it can be speculated that using fewer Ag NPs in the detection could improve the sensitivity, because at low Hg(2+) concentration the interaction becomes stronger since every Ag nanoparticle acts with more Hg(2+) ions. Accordingly, we establish that 90.9 pM (18.2 ppt) Hg(2+) is detected in 18 μM Ag NPs, which is much lower than that in reported papers.