A Label-Free and Ultrasensitive Prussian Blue-Based Dipstick Sensor for Bacterial and Biofilm Detection

Water and food contamination has become the major contributor to infections and deaths. However, rapid and sensitive bacterial detection still remains an unmet demand that has attracted widespread attention. Often water and food samples are sent out for laboratory testing to detect the presence of contamination, which is time-consuming and laborious. Herein, we have developed a highly sensitive, tenable, affordable, and robust (STAR) paper-based colorimetric dipstick sensor based on the principle of Prussian blue (PB) synthesis as an indicator of bacterial contamination. In the presence of bacteria, it leads to the formation of PB, a dye that acts as a colorimetric indicator. The intensity of the PB is the direct measure of the degree of contamination. The fabrication of the STAR dipstick sensor involves a simple and cost-effective process. The STAR dipstick sensor is ultrasensitive and can detect up to 101 CFU/mL of bacteria within minutes of contact with the test sample. The STAR dipstick sensor is fabricated using biodegradable components, which is speculated to facilitate quick and environmentally friendly degradation after each use. The sensor has been validated for its properties and capabilities at different pH to detect both Gram-positive and Gram-negative bacterial strains in real-time samples. The stability and degradation were also monitored. Comprehensively, the proposed STAR dipstick sensor can serve as a point-of-care device to detect bacterial contamination in a swift and sensitive manner.

Goldilocks Energy Minimum: Peptide-Based Reversible Aggregation and Biosensing

Colorimetric biosensors based on gold nanoparticle (AuNP) aggregation are often challenged by matrix interference in biofluids, poor specificity, and limited utility with clinical samples. Here, we propose a peptide-driven nanoscale disassembly approach, where AuNP aggregates induced by electrostatic attractions are dissociated in response to proteolytic cleavage. Initially, citrate-coated AuNPs were assembled via a short cationic peptide (RRK) and characterized by experiments and simulations. The dissociation peptides were then used to reversibly dissociate the AuNP aggregates as a function of target protease detection, i.e., main protease (Mpro), a biomarker for severe acute respiratory syndrome coronavirus 2. The dissociation propensity depends on peptide length, hydrophilicity, charge, and ligand architecture. Finally, our dissociation strategy provides a rapid and distinct optical signal through Mpro cleavage with a detection limit of 12.3 nM in saliva. Our dissociation peptide effectively dissociates plasmonic assemblies in diverse matrices including 100% human saliva, urine, plasma, and seawater, as well as other types of plasmonic nanoparticles such as silver. Our peptide-enabled dissociation platform provides a simple, matrix-insensitive, and versatile method for protease sensing.

Novel rapid detection of melamine based on the synergistic aggregation of gold nanoparticles

A simple and rapid colorimetric method for the detection of melamine in milk samples is described. Polythymidine oligonucleotide was adsorbed on to the surface of gold nanoparticles (AuNPs), protecting it from aggregation. In the presence of melamine, polythymidine oligonucleotide combined with melamine formed a double-strand DNA-like structure, allowing AuNPs aggregation. In the presence of positively charged SYBR Green I (SG I), AuNPs were further aggregated. In the presence of melamine and SG I, aggregation of AuNPs was synergistic. Thus, in this principle, melamine can be detected visually. Plasmon resonance peak changes enabled detection of melamine quantitatively using UV-vis spectroscopy. The limit of detection for this colorimetric method was 16 μg L^-1 with a good linear range from 19.5 μg L^-1 to 1.25 × 10³ μg L^-1, and detection took only 1 min. The method was successfully applied for detection of melamine in milk samples.

Biodegradable and Flexible Thermoplastic Composite Graphite Electrodes: A Promising Platform for Inexpensive and Sensitive Electrochemical Detection of Creatine Kinase at the Point-of-Care

Acute myocardial infarction (AMI) is the main cause of death worldwide, and the time of diagnosis is decisive for the effectiveness of the treatment of patients with AMI. Creatine kinase-myocardial band (CK-MB) has a predominance and high affinity with myocardial tissue, making it considered one of the main biomarkers for the diagnosis of AMI. In this work, we report a novel biodegradable composite material based on a polymer blend of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and Poly(butylene adipate-co-terephthalate) (PHBV:Ecoflex) and graphite microparticles for sensitive and selective electrochemical detection of CK-MB. The morphological and physicochemical characterizations of the thermoplastic composite material revealed a homogeneous and synergistic distribution of the graphite microparticles through the blend structure, providing low defects and high electrical conductivity with high electron transfer kinetics (k⁰ = 3.54 × 10^-3 cm s^-1) features with adequate flexibility for point-of-care applications. The portable and disposable devices were applied to detect CK-MB using the electrochemical impedance spectroscopy (EIS) technique in a relevant clinical concentration ranging from 5.0 ng mL^-1 to 100.0 ng mL^-1 and presented a limit of detection of 0.26 ng mL^-1 CK-MB. The selectivity of the sensor was confirmed by testing the potential interference of major biomolecules found in biofluids and other relevant macromolecules. The accuracy and robustness were assessed by addition and recovery protocol in urine and saliva samples without sample pretreatment and demonstrated the potential of our method for rapid and decentralized tests of AMI. In addition, the study of the thermal, biological, and photodegradation of the devices after being used was also carried out, aiming at the disposal of the material more sustainably.

Di-Arginine Additives for Dissociation of Gold Nanoparticle Aggregates: A Matrix-Insensitive Approach with Applications in Protease Detection

We report the reversible aggregation of gold nanoparticles (AuNPs) assemblies via a di-arginine peptide additive and thiolated PEGs (HS-PEGs). The AuNPs were first aggregated by attractive forces between the citrate-capped surface and the arginine side chains. We found that the HS-PEG thiol group has a higher affinity for the AuNP surface, thus leading to redispersion and colloidal stability. In turn, there was a robust and obvious color change due to on/off plasmonic coupling. The assemblies' dissociation was directly related to the HS-PEG structural properties such as their size or charge. As an example, HS-PEGs with a molecular weight below 1 kDa could dissociate 100% of the assemblies and restore the exact optical properties of the initial AuNP suspension (prior to the assembly). Surprisingly, the dissociation capacity of HS-PEGs was not affected by the composition of the operating medium and could be performed in complex matrices such as plasma, saliva, bile, urine, cell lysates, or even seawater. The high affinity of thiols for the gold surface encompasses by far the one of endogenous molecules and is thus favored. Moreover, starting with AuNPs already aggregated ensured the absence of a background signal as the dissociation of the assemblies was far from spontaneous. Remarkably, it was possible to dry the AuNP assemblies and solubilize them back with HS-PEGs, improving the colorimetric signal generation. We used this system for protease sensing in biological fluids. Trypsin was chosen as the model enzyme, and highly positively charged peptides were conjugated to HS-PEG molecules as cleavage substrates. The increase of positive charge of the HS-PEG-peptide conjugate quenched the dissociation capacity of the HS-PEG molecules, which could only be restored by the proteolytic cleavage. Picomolar limit of detection was obtained as well as the detection in saliva or urine.

RGB colorimetric method based detection of oxytocin in food samples using cysteamine functionalized gold nanoparticles

In the present investigation, a colorimetric method has been demonstrated for detection of oxytocin using Cysteamine functionalized gold nanoparticles (Cys-AuNP). The analyte oxytocin which finds its use in medical field is purposefully injected in fruits and vegetables for their faster growth. Also, it is used in the milk letdown in cattles. The complexity of segregating oxytocin induced fruit and vegetables from naturally grown ones led to development of the method. The choice of spherical gold nanoparticles for development of sensor was influenced from its unique optical properties. Moreover, functionalization with cysteamine increased its sensitivity towards oxytocin detection. The method was coupled with RGB colorimetric technique which exhibited excellent linear correlation between Oxytocin concentration and RGB color values (R² = 0.96). The RGB method used measured the intensity of color and a relation was established between measured RGB values and concentration of analyte (Oxytocin) present in the sample. This formed the basis for generation of simple colorimetric card that can be used to relate the color with amount present in the sample. More significantly, the Cys-AuNP shows excellent selectivity towards other coexisting substances present in the sample. This method with good precision (RSD>15%) offers suitability for onsite application without need for complex instrumentation.

Two-step aggregation of gold nanoparticles based on charge neutralization for detection of melamine by colorimetric and surface-enhanced Raman spectroscopy platform

A colorimetric and surface-enhanced Raman scattering (SERS) signal amplification platform based on 2-step aggregation of gold nanoparticles (AuNP) was constructed for the sensitive detection of melamine. In this study, the positively charged SYBR Green I was used for the first step of aggregation of AuNP, via charge neutralization, to obtain small-sized AuNP aggregates. The positively charged SYBR Green I decreased the negative charges of the surface of AuNP, which was beneficial to the aggregation of AuNP. In addition, the melamine could aggregate AuNP by decreasing the negative charges of the surface of AuNP and self-assemble with each other on the surface of AuNP by hydrogen bonds. Therefore, the second efficient aggregation of small-sized AuNP aggregates could be achieved with melamine at low concentration, resulting in significant signal changes of color and SERS. The sensitivity of a colorimetric (0.60 mg/L) and SERS (0.089 mg/L) platform, based on 2-step aggregation of AuNP, was 15 and 2.2 times higher than that based on 1-step aggregation of AuNP for detecting melamine.

Low-Cost Optodiagnostic for Minute-Time Scale Detection of SARS-CoV-2

The COVID-19 pandemic has exacerbated our society's tremendous health equity gap. Disadvantaged populations have been disproportionally affected by COVID-19, lacking access to affordable testing, a known effective tool for preventing viral spread, hospitalizations, and deaths. Here, we describe COVID-19 Low-cost Optodiagnostic for Rapid testing (COLOR), a colorimetric biosensor fabricated on cotton swabs using gold nanoparticles modified with human angiotensin-converting enzyme 2 (ACE2), which costs 15¢ to produce and detects SARS-CoV-2 within 5 min. COLOR detected very low viral particle loads (limit of detection: 0.154 pg mL-1 of SARS-CoV-2 spike protein), and its color intensity correlated with the cycle threshold (Ct) values obtained using reverse transcription polymerase chain reaction (RT-PCR). The performance of COLOR was assessed using 100 nasopharyngeal/oropharyngeal (NP/OP) clinical samples, yielding sensitivity, specificity, and accuracy values of 96%, 84%, and 90%, respectively. In summary, each COLOR test can be manufactured for 15¢ and presents rapid minute-time scale detection of SARS-CoV-2, thus providing a solution to enable high-frequency testing, particularly in low-resource communities.

Minute-scale detection of SARS-CoV-2 using a low-cost biosensor composed of pencil graphite electrodes

COVID-19 has led to over 3.47 million deaths worldwide and continues to devastate primarily middle- and low-income countries. High-frequency testing has been proposed as a potential solution to prevent outbreaks. However, current tests are not sufficiently low-cost, rapid, or scalable to enable broad COVID-19 testing. Here, we describe LEAD (Low-cost Electrochemical Advanced Diagnostic), a diagnostic test that detects severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) within 6.5 min and costs $1.50 per unit to produce using easily accessible and commercially available materials. LEAD is highly sensitive toward SARS-CoV-2 spike protein (limit of detection = 229 fg⋅mL-1) and displays an excellent performance profile using clinical saliva (100.0% sensitivity, 100.0% specificity, and 100.0% accuracy) and nasopharyngeal/oropharyngeal (88.7% sensitivity, 86.0% specificity, and 87.4% accuracy) samples. No cross-reactivity was detected with other coronavirus or influenza strains. Importantly, LEAD also successfully diagnosed the highly contagious SARS-CoV-2 B.1.1.7 UK variant. The device presents high reproducibility under all conditions tested and preserves its original sensitivity for 5 d when stored at 4 °C in phosphate-buffered saline. Our low-cost and do-it-yourself technology opens new avenues to facilitate high-frequency testing and access to much-needed diagnostic tests in resource-limited settings and low-income communities.

Rapid and sensitive SERS detection of melamine in milk using Ag nanocube array substrate coupled with multivariate analysis

In this study, a facile Ag nanocube (NC) array substrate was fabricated for rapid SERS detection of melamine in milk. This easily-prepared substrate exhibited high Raman enhancement factor (~1.02 × 105) and good reproducibility with ~10.75% spot-to-spot variation in Raman intensity. Our proposed method can detect melamine as low as 0.01 ppm in standard solutions and 0.5 ppm in real milk samples after a simple one-step solvent extraction. Two multivariate analysis tools including partial least squares and support vector machines (SVM) were explored to develop reliable regression models for quantitative SERS analysis of melamine. By comparison, SVM regression models exhibited better predictive performance, especially in liquid milk, with root mean square error (RMSE) of calibration = 5.5783, coefficient of determination (R2) of calibration = 0.9807, RMSE of prediction = 1.9636, and R2 of prediction = 0.9736. Hence, this study offers a rapid and sensitive detection of adulterant melamine in milk samples.

Study of the interaction mechanism between hydrophilic thiol capped gold nanoparticles and melamine in aqueous medium

In the last years, intense efforts have been made in order to obtain colloidal-based systems capable of pointing out the presence of melamine in food samples. In this work, we reported about the recognition of melamine in aqueous solution, using gold nanoparticles stabilized with 3-mercapto-1-propanesulfonate (AuNPs-3MPS), with the aim of deepening how the recognition process works. AuNPs were synthesized using a wet chemical reduction method. The synthesized AuNPs-3MPS probe was fully characterized, before and after the recognition process, by both physicochemical (UV-vis, FT-IR, ¹H-NMR, DLS and ζ-potential) and morphostructural techniques (AFM, HR-TEM). The chemical and electronic structure was also investigated by SR-XPS. The sensing method is based on the melamine-induced aggregation of AuNPs; the presence of melamine was successfully detected in the range of 2.5-500 ppm. The results achieved also demonstrate that negatively charged AuNPs-3MPS are potentially useful for determining melamine contents in aqueous solution. SR-XPS measurements allowed to understand interaction mechanism between the probe and the analyte. The presence of sulfonate groups allows a mutual interaction mediated by electrostatic bonds between nanoparticles surface thiols and positively charged amino groups of melamine molecules.

Aptamer-Facilitated Design of Gold Nanoparticle-Based Logic Gates for Cyromazine and Melamine Detection in Milk

In this paper, we developed a simply designed detection method for logic gates by using aptamers, cetyltrimethyl ammonium bromide, together with melamine and cyromazine to control the aggregation and dispersion of gold nanoparticles (AuNPs). First, either melamine or cyromazine can induce the aggregation of AuNPs, supporting an OR gate to detect whether they were present or not. Second, based on the fact that aptamer T31 can specifically bind with melamine, preventing the aggregation of AuNPs, an improved INHIBIT gate was also fabricated to find whether there was melamine. It has a detection limit of 0.12ppm by the naked eye for the detection of melamine, and the limit of detection (LOD) by spectrophotometer is 2.2ppb. Third, with the adsorption of aptamer Tcy1 on AuNPs and the strong coordination of Tcy1 with cyromazine, the addition of cyromazine and CTAB immediately resulted in the aggregation of AuNPs, giving rise to an AND gate. This gate has a detection limit of 0.17ppm by the naked eye and the limit of detection (LOD) is 9.0ppb by spectrophotometer. The system provided a good platform for the development of functional logic systems.

The effect of pH and transition metal ions on cysteine-assisted gold aggregation for a distinct colorimetric response

Colorimetric detection is a promising sensing strategy that is applicable to qualitative and quantitative determination of an analyte by monitoring visually detectable color changes with the naked eye. This study explored the cysteine (Cys)-induced aggregation of gold nanoparticles (AuNPs) in order to develop a sensitive colorimetric detection method for Cys. For this purpose, we systematically investigated the colorimetric response of AuNPs to Cys with varying particle sizes and concentrations. The AuNPs with various diameters ranging from 26.5 nm to 58.2 nm were synthesized by the citrate reduction method. When dispersed in water to have the same surface area per unit volume, the smaller AuNPs (26.5 nm) exhibited a more sensitive response to Cys compared to a larger counterpart (46.3 nm). We also examined the effect of divalent first-row transition metal ions (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) on the Cys-induced aggregation of AuNPs. Among the tested metal ions, the addition of Cu²⁺ provided the highest enhancement in sensitivity to Cys regardless of pH between 3.5 and 7. The significant increase in the sensitivity caused by Cu²⁺ could be attributed to the capability of Cu²⁺ to form a highly stable chelate complex with surface-immobilized Cys, facilitating the aggregation of AuNPs. For the AuNPs–Cu²⁺ system at pH 7, the detection limit for Cys was determined to be 5 nM using UV-vis spectroscopy. The reported strategy showed the potential to be used for a rapid and sensitive detection of Cys and also metal ions that can facilitate Cys-mediated aggregation of AuNPs.

Divalent transition metal ions facilitated the aggregation of gold nanoparticles: Fe²⁺ < Ni²⁺ < Zn²⁺ < Co²⁺ ≪ Mn²⁺ < Cu²⁺ at pH 7. The optimized AuNPs-Cu²⁺ system produced the progressive color change upon the addition of cysteine (0.2–2.0 μM).

Challenges for cysteamine stabilization, quantification, and biological effects improvement

The aminothiol cysteamine, derived from coenzyme A degradation in mammalian cells, presents several biological applications. However, the bitter taste and sickening odor, chemical instability, hygroscopicity, and poor pharmacokinetic profile of cysteamine limit its efficacy. The use of encapsulation systems is a good methodology to overcome these undesirable properties and improve the pharmacokinetic behavior of cysteamine. Besides, the conjugation of cysteamine to the surface of nanoparticles is generally proposed to improve the intra-oral delivery of cyclodextrin-drug inclusion complexes, as well as to enhance the colorimetric detection of compounds by a gold nanoparticle aggregation method. On the other hand, the detection and quantification of cysteamine is a challenging mission due to the lack of a chromophore in its structure and its susceptibility to oxidation before or during the analysis. Derivatization agents are therefore applied for the quantification of this molecule. To our knowledge, the derivatization techniques and the encapsulation systems used for cysteamine delivery were not reviewed previously. Thus, this review aims to compile all the data on these methods as well as to provide an overview of the various biological applications of cysteamine focusing on its skin application.

Handheld fluorometer for in-situ melamine detection via interference synthesis of dsDNA-templated copper nanoparticles

Fluorescent copper nanoparticles templated by dsDNA have gained significant research interest as they are inexpensive and easy to synthesize, and have found applications in the detection of a wide range of analytes. The presence of the analyte in the reaction mixture interferes with the synthesis of the copper nanoparticles and the subsequent drop in fluorescence can be correlated to the concentration of the analyte present in the solution. Analyte detection using copper nanoparticle-based assays is amenable for in-situ applications as the test does not require expensive reagents and can be performed at room temperature. However, expensive and sophisticated detection systems are required for the detection of copper nanoparticles due to the low fluorescence emission signal from these nanoparticles. This restricts the use of the technology to centralized labs. Utilizing a recently developed chemical technique for fluorescence enhancement, this paper presents the first report of a handheld fluorometer capable of detecting DNA-templated copper nanoparticles. The fluorometer is portable and constructed with low-cost, off-the-shelf components like a UV-LED and a PIN photodiode. The performance of the developed system is demonstrated through the detection of melamine in milk samples via the interference synthesis of copper nanoparticles. Melamine is an adulterant used in dairy products that is harmful to human health if present in levels above 1 ppm. The developed system is capable of detecting up to 0.1 ppm of melamine in milk samples with a linear relationship observed between the detector output and concentration of melamine in the range from 0.1 ppm to 100 ppm (R² = 0.9979).

Synthesis and characterization of maltol capped silver nanoparticles and their potential application as an antimicrobial agent and colorimetric sensor for cysteine

Maltol capped silver nanoparticles (McAgNPs) were synthesized using maltol (3-hydroxy-2-methyl-4-pyrone) as reducing and capping agent. McAgNPs were characterized by Visible and FTIR (Fourier transform infrared) spectroscopy, dynamic light scattering (DLS), and atomic force microscopy (AFM). Bright yellow color McAgNPs showed surface plasmon resonance (SPR) band at 436 nm, spherical shape and the average size between 35 to 50 nm. McAgNPs revealed higher stability against varying storage time, temperature, pH and salt concentrations. McAgNPs were successfully utilized for the selective and highly sensitive colorimetric detection of cysteine (Cys). Addition of Cys in a solution of McAgNPs, resulted a rapid change in color from yellow to orange because of the formation of nanoaggregates as confirmed by Visible/FTIR spectroscopy, DLS, and AFM studies. The estimated limit of detection (0.043 μM) was found to be more sensitive than previously reported other optical methods. The practical applicability of probe was also established by spiking the known concentrations of Cys in biological (blood plasma and urine) and environmental (tap and lake water) samples with significant recovery rates (92-104.6%). Despite being nontoxic to various tested cell lines, McAgNPs demonstrated potent antimicrobial, antibiofilm, and biofilm eradicating activities, thus potentially valuable in diagnostics and/or the synthesis of other nanocomposite material for broader applications.

Hemolysis tendency of anticancer nanoparticles changes with type of blood group antigen: An insight into blood nanoparticle interactions

Different blood groups of ABO system have specific antigen which bestows them with different biochemical properties and hence they can show different hemolytic activity. In this report, hemolytic activity of thiol-functionalized Fe₃O₄-Au nanoparticles were studied in presence and absence of doxorubicin and the effect of various thiol coatings were correlated towards their hemolysis tendency. The nanoparticles were functionalized with four different amino thiols, cysteamine (CEA), cystamine (CA), cysteine (Cys) and cystine (Cyt) to form Fe₃O₄-Au CEA, Fe₃O₄-Au CA, Fe₃O₄-Au Cys and Fe₃O₄-Au Cyt nanoparticles which were loaded with anticancer drug, doxorubicin. The functionalization was characterized using ATR-FTIR, HR-TEM, XPS and other spectroscopic methods. Maximum drug encapsulation efficiency of 83% was observed with Fe₃O₄-Au CA nanoparticles. In-vitro experiments were performed on HeLa cells to check the cellular uptake and cytotoxicity using MTT assay. Hemolytic activity was then analyzed with all the blood groups (positive and negative). The amino acid functionalized, Fe₃O₄-Au Cys and Fe₃O₄-Au Cyt nanoparticles, shows lesser hemolysis compared to amino thiol functionalized Fe₃O₄-Au CEA, and Fe₃O₄-Au CA nanoparticles. In positive blood groups, the Fe₃O₄-Au CA nanoparticles shows the highest rate of hemolysis followed by Fe₃O₄-Au CEA, while the lowest hemolysis rate was observed for Fe₃O₄-Au Cyt nanoparticles. For negative blood groups, the thiol coated nanoparticles show more abrupt hemolysis rate depending upon the type of antigen.

Cysteamine-coated gold nanoparticles for bimodal colorimetric detection with inverse sensitivity: a proof-of-concept with lysozyme

Cysteamine-coated gold nanoparticles (cysAuNPs) are positively charged as-synthesised and hence can interact with negatively charged DNA with ease. We have investigated the dependency of the particles' dispersion stage on different concentrations of lysozyme-binding aptamer (LBA). On top of the commonly reported phenomenon where cysAuNPs aggregate as the concentration of LBA increases, we observed that cysAuNPs redispersed after the amount of LBA achieved a certain threshold, dubbed as the critical redispersion concentration (CRC). By harnessing the aggregation and dispersion behaviour of cysAuNPs at LBA below and above the CRC, respectively, we have demonstrated a bimodal colorimetric aptasensor to detect lysozyme as a proof-of-concept study. Apart from being able to quantify the lysozyme in different ranges of concentrations with a visual change in colour, this aptasensor also demonstrated a novel concept of inverse sensitivity (i.e. higher signal with less analyte), leading to a 24-fold higher of signal-to-noise ratio (SNR), in comparison to the conventional sensors. The aptasensor can also selectively distinguish lysozyme and eliminate false results from other control proteins via both modes. The generalisability, as well as potential of cysAuNPs for bimodal colorimetric detection and inverse sensitivity behaviour have made this material an interesting alternative to citrate-coated AuNPs.

An aptasensor derived from cysteamine-gold nanoparticles for bimodal colorimetric detection and with inverse sensitivity pattern.

Review on Nanomaterial-Based Melamine Detection

Illegal adulteration of milk products by melamine and its analogs has become a threat to the world. In 2008, the misuse of melamine with infant formula caused serious effects on babies of China. Thereafter, the government of China and the US Food and Drug Administration (FDA) limited the use of melamine of 1 mg/kg for infant formula and 2.5 mg/kg for other dairy products. Similarly, the World Health Organization (WHO) has also limited the daily intake of melamine of 0.2 mg/kg body weight per day. Many sensory schemes have been proposed by the scientists for carrying out screening on melamine poisoning. Among them, nanomaterial-based sensing techniques are very promising in terms of real-time applicability. These materials uncover and quantify the melamine by means of diverse mechanisms, such as fluorescence resonance energy transfer (FRET), aggregation, inner filter effect, surface-enhanced Raman scattering (SERS), and self-assembly, etc. Nanomaterials used for the melamine determination include carbon dots, quantum dots, nanocomposites, nanocrystals, nanoclusters, nanoparticles, nanorods, nanowires, and nanotubes. In this review, we summarize and comment on the melamine sensing abilities of these nanomaterials for their suitability and future research directions.

A Rapid and Sensitive Aptasensor for Cyromazine Detection in Raw Milk Based on a Nanogold Probe and G-Quadruplex Formation

Herein, a rapid, facile, and colourimetric sensor for the detection of cyromazine in raw milk is reported using an aptamer based on gold nanoparticles (AuNPs). A sequence-specific aptamer for cyromazine called Tcyr1 is designed to absorb on the surface of AuNPs and electrostatically interacts with poly(diallyldimethylammonium chloride) (PDDA), which prevents AuNPs from aggregating. It can also self-assemble to form a G-quadruplex-CYR complex with cyromazine. Because of its specificity and stability, the introduction of cyromazine in raw milk would influence the protection thus the following cationic polymer could aggregate AuNPs and cause a remarkable change in colour. According to this, the presence of cyromazine can be determined by the naked eye and means of absorbance. This sensor is selective for the detection of cyromazine in raw milk and has a limit of detection of 200 ppb by the naked eye and of 5.8 ppb by spectrophotometer, and has a detection range from 0.1 to 1 ppm.

Internal Calibration Potentiometric Aptasensors for Simultaneous Detection of Hg2+, Cd2+, and As3+ Based on a Screen-Printed Carbon Electrodes Array

An all-solid-state potentiometric aptasensor array based on a multichannel disposable screen-printed carbon electrode (SPCE) was demonstrated for the simultaneous detection of Hg²⁺, Cd²⁺, and As³⁺ by open circuit potential (OCP) technology. The potential of the channel with an internal calibration DNA sequence (IC-DNA) was employed as the internal calibration potential (ICP) to subtract the background signal generated by the detection system, providing a built-in correction methodology. As a result, the developed aptasensor array showed high sensitivity and accuracy for detecting Hg²⁺, Cd²⁺, and As³⁺ without mutual interference or interference from other ions. The linear response ranged from 2.5 pM to 2.5 μM, and the detection limits for Hg²⁺, Cd²⁺, and As³⁺ were 2.0, 0.62, and 0.17 pM, respectively. Furthermore, the potentiometric aptasensor array was successfully applied for the simultaneous detection of three ions in real samples. The results obtained from the developed approach agreed well with the results obtained from inductively coupled plasma mass spectrometry.

Cysteamine-Modified Gold Nanoparticles as a Colorimetric Sensor for the Rapid Detection of Gentamicin

A simple, rapid, and specific colorimetric method for gentamicin detection using cysteamine-modified gold nanoparticles (cys-AuNPs) has been developed. The maximum residue limits of gentamicin allowed in foods are typically below 100 nM, so an effective detection method for low concentrations of the drug is required. The aggregation of gold nanoparticles (AuNPs) was used as the basis for this method, and adding cysteamine to the AuNPs helped to enhance their aggregative abilities. The cys-AuNPs are capable of detecting gentamicin concentrations as low as 12.45 nM in water, which could be quantified using UV-vis spectroscopy. Samples extracted from skim milk with a simple pretreatment showed that gentamicin concentrations down to at least 100 nM could be observed using the cys-AuNPs. This study demonstrates the ability of the cys-AuNPs to rapidly detect and quantify gentamicin in both simple and complex matrices.

PRACTICAL APPLICATION

This study demonstrates that cysteamine-modified gold nanoparticles could be used as a rapid and efficient tool for gentamicin detection. This technique is cheaper, simpler, and more effective than many other methods that are currently used for detecting the antibiotic in industrial and commercial applications. It has a great potential to be practically applied as a rapid screening method for gentamicin and gentamicin-like compounds in food and environmental samples.

A Plasmonic Mass Spectrometry Approach for Detection of Small Nutrients and Toxins

Nutriology relies on advanced analytical tools to study the molecular compositions of food and provide key information on sample quality/safety. Small nutrients detection is challenging due to the high diversity and broad dynamic range of molecules in food samples, and a further issue is to track low abundance toxins. Herein, we developed a novel plasmonic matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) approach to detect small nutrients and toxins in complex biological emulsion samples. Silver nanoshells (SiO₂@Ag) with optimized structures were used as matrices and achieved direct analysis of ~ 6 nL of human breast milk without any enrichment or separation. We performed identification and quantitation of small nutrients and toxins with limit-of-detection down to 0.4 pmol (for melamine) and reaction time shortened to minutes, which is superior to the conventional biochemical method currently in use. The developed approach contributes to the near-future application of MALDI MS in a broad field and personalized design of plasmonic materials for real-case bio-analysis.

Visual and Colorimetric Sensing of Metsulfuron-Methyl by Exploiting Hydrogen Bond-Induced Anti-Aggregation of Gold Nanoparticles in the Presence of Melamine

Various highly sensitive and selective analytical methods have been used to monitor metsulfuron-methyl residue in the environment. However, these methods involve costly instruments and complex, time-consuming operations performed in laboratories. Here, a rapid, convenient, and sensitive colorimetric sensor based on anti-aggregation of gold nanoparticles (AuNPs) is demonstrated for the rapid detection of metsulfuron-methyl in agricultural irrigation water. The AuNPs could be induced to aggregate in the presence of melamine and exhibited a distinct color change from wine-red to blue. The aggregation was suppressed by a strong hydrogen-bonding interaction between metsulfuron-methyl and melamine. The differences of the absorbance at 523 nm (ΔA523) and the color change was linearly related to metsulfuron-methyl concentration over the range 0.1⁻100 mg/L, as observed visually and by UV-vis (Ultraviolet-visible) spectrometry. The detection limit of the sensor was as low as 0.05 mg/L (signal/noise = 3), and was used to determine metsulfuron-methyl in spiked water and in agricultural irrigation water samples. Recoveries were in the range of 71.2⁻100.4%, suggesting that the colorimetric sensor was suitable for the determination of metsulfuron-methyl in agricultural water samples.

A Sensitive "Turn-On" Fluorescent Sensor for Melamine Based on FRET Effect between Polydopamine-Glutathione Nanoparticles and Ag Nanoparticles

In this work, Ag nanoparticles (AgNPs) were synthesized quickly by a one-step method utilizing polydopamine-glutathione nanoparticles (PDA-GNPs) as a reducing agent. The PDA-GNPs and the generated AgNPs acted as the energy donor and acceptor, respectively. Accordingly, the fluorescence of PDA-GNPs was quenched on the basis of fluorescence resonance energy transfer (FRET). In the presence of melamine, the preferential combination of Ag(I) and melamine to form Ag(I)-melamine complex prevents Ag(I) from forming AgNPs, together with fluorescence enhancement compared with the absence of melamine. Under the optimal conditions including the concentration of AgNO₃, reaction time, reaction temperature, and pH, the fluorescence enhancement efficiency has a linear response to the concentration of melamine from 0.1 to 40 μM with a detection limit of 23 nM for melamine. The proposed method is simple, time-saving, and low-cost, which was further applied to detect melamine in real milk products with satisfactory results.

Enhanced Sensitivity to Detection Nanomolar Level of Cu2+ Compared to Spectrophotometry Method by Functionalized Gold Nanoparticles: Design of Sensor Assisted by Exploiting First-order Data with Chemometrics

A simple, sensitive and efficient colorimetric assay platform for the determination of Cu²⁺ was proposed with the aim of developing sensitive detection based on the aggregation of AuNPs in presence of a histamine H2-receptor antagonist (famotidine, FAM) as recognition site. This study is the first to demonstrate that the molar extinction coefficients of the complexes formed by FAM and Cu²⁺ are very low (by analyzing the chemometrics methods on the first order data arising from different metal to ligand ratio method), leading to the undesirable sensitivity of FAM-based assays. To resolve the problem of low sensitivity, the colorimetry method based on the Cu²⁺-induced aggregation of AuNPs functionalized with FAM was introduced. This procedure is accompanied by a color change from bright red to blue which can be observed with the naked eyes. Detection sensitivity obtained by the developed method increased about 100 fold compared with the spectrophotometry method. This sensor exhibited a good linear relation between the absorbance ratios at 670 to 520nm (A₆₇₀/₅₂₀) and the concentration in the range 2-110nM with LOD=0.76nM. The satisfactory analytical performance of the proposed sensor facilitates the development of simple and affordable UV-Vis chemosensors for environmental applications.

A simple fluorescent assay for cyromazine detection in raw milk by using CYR-stabilized G-quadruplex formation

A rapid biosensor for the detection of cyromazine in milk is reported based on a fluorescence quenching result. When an FAM labelled G-rich ssDNA Tcy2 is treated with cyromazine, it can form a G-quadruplex-CYR complex and cause a change in fluorescence. As a result, the presence of cyromazine can be determined by fluorescence quenching. This sensor is selective for the detection of cyromazine in raw milk and has a limit of detection of 0.68 ppb and a detection range from 0 to 200 ppb.

A rapid biosensor for the detection of cyromazine in milk is reported based on a fluorescence quenching result.

Triplex DNA-based Bioanalytical Platform for Highly Sensitive Homogeneous Electrochemical Detection of Melamine

Melamine detection has attracted much attention since the discovery of the damage of melamine to human health. Herein, we have developed a sensitive homogeneous electroanalytical platform for melamine detection, which is relied on the formation of triplex molecular beacon integrated with exonuclease III (Exo III)-mediated signal amplification. The formation of triplex molecular beacon was triggered by the recognition and incorporation of melamine to the abasic (AP) site contained in the triplex stem. The stem of the triplex molecular beacon was designed to have a protruding double-strand DNA, which can be recognized and hydrolyzed by Exo III for releasing methylene blue (MB)-labeled mononucleotide. These released MB molecules exhibit high diffusivity toward indium tin oxide electrode with negative charge, thus producing a significantly increased electrochemical response. Taking advantages of the high binding affinity of the DNA triplex structure containing AP sites towards melamine and the unique features of Exo III, this sensing platform is capable for sensitive and selective melamine assay with a detection limit as low as 8.7 nM. Furthermore, this strategy shows good applicability for melamine assay in real samples. Therefore, this strategy broadens the application of triplex DNA and presents a new method for sensitive detection of melamine.

Colorimetric sensors for rapid detection of various analytes

Sensor technology for the rapid detection of the analytes with high sensitivity and selectivity has several challenges. Despite the challenges, colorimetric sensors have been widely accepted for its high sensitive and selective response towards various analytes. In this review, colorimetric sensors for the detection of biomolecules like protein, DNA, pathogen and chemical compounds like heavy metal ions, toxic gases and organic compounds have been elaborately discussed. The visible sensing mechanism based on Surface Plasmon Resonance (SPR) using metal nanoparticles like Au, Ag, thin film interference using SiO₂ and colorimetric array-based technique have been highlighted. The optical property of metal nanoparticles enables a visual color change during its interaction with the analytes owing to the dispersion and aggregation of nanoparticles. Recently, colorimetric changes using silica substrate for detection of protein and small molecules by thin film interference as a visible sensing mechanism has been developed without the usage of fluorescent or radioisotopes labels. Multilayer of biomaterials were used as a platform where reflection and interference of scattering light occur due to which color change happens leading to rapid sensing. Colorimetric array-based technique for the detection of organic compounds using chemoresponsive dyes has also been focused wherein the interaction of the analytes with the substrate coated with chemoresponsive dyes gives colorimetric change.

Biological response of HeLa cells to gold nanoparticles coated with organic molecules

In this work, gold nanospheres functionalized with low weight organic molecules (4-aminothiphenol and cysteamine) were synthesized in a one-step method for their in vitro cytotoxic evaluation on HeLa cells. To enhance the biocompatibility of the cysteamine-capped GNPs, BSA was used due to its broad PH stability and high binding affinity to gold nanoparticles. Besides, the widely reported silica coated gold nanorods were tested here to contrast their toxic response against our nanoparticles coated with organic molecules. Our results shown, the viability measured at 1.9×10^-5M did not show significant differences against negative controls for all the samples; however, the metabolic activity of HeLa cells dropped when they were exposed to silica gold nanorods in the range of concentrations from 2.9×10^-7M to 3.0×10^-4M, while in the cases of gold nanospheres, we found that only at concentrations below 1.9×10^-5M metabolic activity was normal. Our preliminary results did not indicate any perceivable harmful toxicity to cell membrane, cytoskeleton or nucleus due to our nanospheres at 1.9×10^-5M. Additional test should be conducted in order to ensure a safe use of them for biological applications, and to determine the extent of possible damage.

Large-scale preparation of flexible and reusable surface-enhanced Raman scattering platform based on electrospinning AgNPs/PCL nanofiber membrane

A flexible and reusable SERS substrates were prepared by electrospinning Ag nanoparticles in reversed micelle into poly(ε-caprolactone) nanofibers.

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.

Surface-enhanced Raman scattering substrate based on cysteamine-modified gold nanoparticle aggregation for highly sensitive pentachlorophenol detection

A new surface-enhanced Raman scattering (SERS) substrate based on cysteamine-modified gold nanoparticles (AuNPs) on the glass surface has been developed for the monitoring of pentachlorophenol (PCP) in water samples.