Gold nanoparticle/carbon nanotube hybrids as an enhanced material for sensitive amperometric determination of tryptophan

Click-Functionalization of Silanized Carbon Nanotubes: From Inorganic Heterostructures to Biosensing Nanohybrids

Here we present an approach to functionalize silanized single-walled carbon nanotubes (SWNTs) through copper-free click chemistry for the assembly of inorganic and biological nanohybrids. The nanotube functionalization route involves silanization and strain-promoted azide-alkyne cycloaddition reactions (SPACC). This was characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and Fourier transform infra-red spectroscopy. Silane-azide-functionalized SWNTs were immobilized from solution onto patterned substrates through dielectrophoresis (DEP). We demonstrate the general applicability of our strategy for the functionalization of SWNTs with metal nanoparticles (gold nanoparticles), fluorescent dyes (Alexa Fluor 647) and biomolecules (aptamers). In this regard, dopamine-binding aptamers were conjugated to the functionalized SWNTs to perform real-time detection of dopamine at different concentrations. Additionally, the chemical route is shown to selectively functionalize individual nanotubes grown on the surface of silicon substrates, contributing towards future nano electronic device applications.

A New Electrochemical Method to Determine Tryptophan in Fruit Juices: Development and Validation

Tryptophan (Trp) is an essential amino acid usually found in fruit juices. Its determination is necessary for food companies because of its relation to human health. In this work, a new electrochemical method based on sonogel–carbon electrodes (SNGCEs) was developed and validated using an ultra performance liquid chromatography (UPLC) method as a reference method for the determination of Trp in fruit juices. Cyclic voltammetry (CV), chronoamperometry, and differential pulse voltammetry (DPV) techniques were applied to investigate the oxidation of Trp on a previously polarized SNGCE surface in a Britton–Robinson (BR) buffer solution at pH 3.6. The operating conditions for electroanalysis were optimized using a Box–Behnken design (BBD), obtaining an oxidation peak for Trp at 0.749 V. The linear range for this method was from 0.1 to 5 mg/L. The intraday and interday precision, expressed as a relative standard deviation (RSD), were 3.1% and 2.7%, respectively. The average recovery was 99.01%, and the limit of detection and quantitation were 0.33 and 1.09 mg/L, respectively. Therefore, from the quality analytical parameters obtained, it can be concluded that the new electrochemical method can be successfully used for the routine analysis of Trp in fruit juices. As far as we are concerned, this is the first time that a methodology for Trp determination was performed in this kind of real food matrices.

Electrochemical Amino Acid Sensing: A Review on Challenges and Achievements

The rapid growth of research in electrochemistry in the last decade has resulted in a significant advancement in exploiting electrochemical strategies for assessing biological substances. Among these, amino acids are of utmost interest due to their key role in human health. Indeed, an unbalanced amino acid level is the origin of several metabolic and genetic diseases, which has led to a great need for effective and reliable evaluation methods. This review is an effort to summarize and present both challenges and achievements in electrochemical amino acid sensing from the last decade (from 2010 onwards) to show where limitations and advantages stem from. In this review, we place special emphasis on five well-known electroactive amino acids, namely cysteine, tyrosine, tryptophan, methionine and histidine. The recent research and achievements in this area and significant performance metrics of the proposed electrochemical sensors, including the limit of detection, sensitivity, stability, linear dynamic range(s) and applicability in real sample analysis, are summarized and presented in separate sections. More than 400 recent scientific studies were included in this review to portray a rich set of ideas and exemplify the capabilities of the electrochemical strategies to detect these essential biomolecules at trace and even ultra-trace levels. Finally, we discuss, in the last section, the remaining issues and the opportunities to push the boundaries of our knowledge in amino acid electrochemistry even further.

Mass Spectrometry Imaging of Low-Molecular-Weight Phenols Liberated from Plastics

The abundant and heterogeneous distribution of toxic phenol from plastics has drawn worldwide attention. However, the common analysis methods failed to identify the accurate species of these phenolic hazards from plastics in a direct and nondestructive approach. Herein, we demonstrate the layered double hydroxides (LDHs) as a novel matrix in matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) for low-molecular-weight phenols leaked from plastics. LDHs own abundant hydroxyl groups to facilitate chemoselectivity and ionization of phenols through the formation of hydrogen bonds with these phenols. More importantly, the LDH matrix could provide a distinguishable signal for the homolog and isomeride of these phenolic hazards. The developed method could realize nondestructive and in situ mapping of phenolic hazards in plastics. Our success could help to track the low-molecular-weight compounds liberated from plastics and supply spatial information for polluted plastics. We anticipated that the proposed approach could provide sufficient information to evaluate and alarm the safety of food packaging plastics.

Nitrogen Vacancy Engineering in Graphitic Carbon Nitride for Strong, Stable, and Wavelength Tunable Electrochemiluminescence Emissions

As an attractive electrochemiluminescence (ECL) emitter, graphitic carbon nitride (CN) still suffers from weak and unstable ECL signals for its poor conductivity and the occurrence of electrode passivation. In this study, a simple nitrogen vacancy (NV) engineering strategy has been developed for the improvement of ECL performances (intensity and stability) for the first time. In comparison to pristine CN (RSD = 51.98% for 10 continuous scan), ca. 60 times amplification in ECL intensity and 70 times enhancement in ECL efficiency for CN modified with NVs (CN-NVs) were obtained. In addition, more stable ECL emissions (RSD = 0.53%) were achieved for CN-NV-550 by thermal treatment of pristine CN in a N₂ atmosphere for another 2 h at 550 °C. The mechanism study for the vital role of NVs on the ECL of CN-NVs revealed that NVs can not only facilitate electron transfer to amplify the ECL intensity but also serve as the electron trap to inhibit electrode passivation. More interestingly, a series of CN-NVs exhibited a tunable ECL wavelength range from 470 to 516 nm with different NV contents. Moreover, their ECL spectra showed an obvious red-shift of the wavelength with their corresponding fluorescence spectra. These findings confirmed that the ECL emissions of CN-NVs were susceptible to the relevant surface states of NVs. Our work may open up a promising pathway for improving ECL performances of CN and create new possibilities for multitarget simultaneous detection based on ECL and construction of color tunable light-emitting devices.

A new hybrid nanocomposite electrode based on Au/CeO2-decorated functionalized glassy carbon microspheres for the voltammetric sensing of quercetin and its interaction with DNA

A new hybrid composite containing cerium oxide nanoparticle (CeO2NP) and gold nanoparticle (AuNP)-decorated functionalized glassy carbon microspheres (FGCM) was synthesized (Au/CeO2@FGCM). As a result, an Au/CeO2@FGCM-paraffin oil paste electrode (PE) (Au/CeO2@FGCM-PE) was fabricated and employed for the voltammetric sensing of quercetin (QRT). The structure and surface morphology of Au/CeO2@FGCM were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cyclic voltammetry (CV), square wave voltammetry (SWV) and electrochemical impedance spectroscopy (EIS) were employed for the investigation of the electrochemical behavior of Au/CeO2@FGCM-PE. Under the optimum conditions, the SWV oxidation peak current showed linear dependence on the QRT concentration in the range from 48 nM to 1.09 μM. The achieved limits of detection and quantitation were 0.37 nM and 1.22 nM, respectively. Au/CeO2@FGCM-PE was reproducible, sensitive and stable and displayed anti-interference ability for various common interferents. The proposed method was also successfully applied for real sample analysis. The QRT content extracted from natural sources was determined, and satisfactory results were achieved. Furthermore, the interaction of QRT with salmon testes and calf thymus dsDNA (st-DNA and ct-DNA) on Au/CeO2@FGCM-PE was studied by CV and SWV. The corresponding binding constant (K), surface concentration (Γ), and Gibbs free energy (ΔG°) were computed for the free QRT and the bound QRT-dsDNA complex. The calculated K values for the QRT-ct-DNA and QRT-st-DNA complexes were found to be 6.24 × 105 M-1 and 3.63 × 105 M-1, respectively, which revealed that QRT strongly interacted with ct-DNA compared to that with st-DNA. The decreased intensity of the QRT oxidation peak resulting from its interaction with dsDNA provides a chance to use QRT as a new indicator to analyze ct-DNA and st-DNA.

A screen printed carbon electrode modified with carbon nanotubes and gold nanoparticles as a sensitive electrochemical sensor for determination of thiamphenicol residue in milk

Antibiotic residues in milk are of great concern for health regulatory agencies, milk consumers, and dairy farmers due to their destructive effects, ranging from allergic reactions, antibiotic resistance and the ability to interfere with the production of fermented products (i.e. cheese and yogurt). Therefore, a reliable, fast, and simple method needs to be developed to monitor antibiotic residues in milk samples before distribution to consumers. In this study, the first sensitive electrochemical sensor is presented for the determination of thiamphenicol (TAP), a broad-spectrum antibiotic in bovine milk. In the fabrication process, a screen printed electrode (SPE) was modified with gold nanoparticles (AuNPs) and carbon nanotubes (CNTs) using ethylenediamine (en) as a cross linker. Cyclic voltammetry studies showed an adsorptive control process for the electro-oxidation of TAP at −0.1 V on the modified electrode of SPE/CNT/en/AuNPs. Differential pulse voltammetry (DPV) was applied for the quantitative determination of TAP under optimized conditions (0.1 M citrate buffer, pH 6.0, accumulation potential −0.7 V, and accumulation time 150 s). A DPV study for TAP shows a wide linear calibration range of 0.1–30 μM with the detection limit of 0.003 μM. Furthermore, the developed sensor displays high sensitivity, reproducibility, repeatability, and good stability for the detection of TAP. The proposed sensor was successfully applied for the determination of spiked TAP in bovine milk with satisfactory results.

The first portable electrochemical sensor was constructed for monitoring of thiamphenicol residue in fresh milk based on electro-oxidation of thiamphenicol on the surface of CNTs and AuNPs.

Electrochemical and optical bimodal sensing of caffeic acid based on electrodes made from nanorods of AuNPs:PEDOT:PSS and bio-hybrid chitosan:PEDOT:PSS

One pot in situ synthesis of AuNPs:PEDOT:PSS using a green solvent and its utilization as an electrochemical sensor.

A tryptophan assay based on the glassy carbon electrode modified with a nano-sized tryptophan-imprinted polymer and multi-walled carbon nanotubes

A new nanostructured molecularly imprinted polymer (nano-MIP), possessing tryptophan compatible cavities, was synthesized using 1-(allyloxy)-4-nitrobenzene as an innovative functional monomer.

Construction of an Electrochemical Sensor Based on Carbon Nanotubes/Gold Nanoparticles for Trace Determination of Amoxicillin in Bovine Milk

In this work, a novel electrochemical sensor was fabricated for determination of amoxicillin in bovine milk samples by decoration of carboxylated multi-walled carbon nanotubes (MWCNTs) with gold nanoparticles (AuNPs) using ethylenediamine (en) as a cross linker (AuNPs/en-MWCNTs). The constructed nanocomposite was homogenized in dimethylformamide and drop casted on screen printed electrode. Field emission scanning electron microscopy (FESEM), energy dispersive X-Ray (EDX), X-Ray diffraction (XRD) and cyclic voltammetry were used to characterize the synthesized nanocomposites. The results show that the synthesized nanocomposites induced a remarkable synergetic effect for the oxidation of amoxicillin. Effect of some parameters, including pH, buffer, scan rate, accumulation potential, accumulation time and amount of casted nanocomposites, on the sensitivity of fabricated sensor were optimized. Under the optimum conditions, there was two linear calibration ranges from 0.2-10 µM and 10-30 µM with equations of Ipa (µA) = 2.88C (µM) + 1.2017; r = 0.9939 and Ipa (µA) = 0.88C (µM) + 22.97; r = 0.9973, respectively. The limit of detection (LOD) and limit of quantitation (LOQ) were calculated as 0.015 µM and 0.149 µM, respectively. The fabricated electrochemical sensor was successfully applied for determination of Amoxicillin in bovine milk samples and all results compared with high performance liquid chromatography (HPLC) standard method.

Reusable potentiometric screen-printed sensor and label-free aptasensor with pseudo-reference electrode for determination of tryptophan in the presence of tyrosine

Analysis of L-tryptophan (Trp) in biological samples has great importance for biomedical studies. Amino acid tyrosine (Tyr) that usually coexist with Trp in biological fluids can significantly interfere with reliable determination of Trp. In the current study, we demonstrate the development of two ultra-sensitive electrochemical sensor and label-free aptasensor for selective analysis of Trp in biological samples (i.e., cow's milk and human plasma, saliva and urine samples). In addition, without using AgCl/KCl, an Ag pseudo-reference screen printed electrode (Ag-PR-SPE) was exploited as a reference electrode. To prepare the engineered Trp sensor/aptasensor, a gold SPE was first modified with multiwall carbon nanotube (MWCNT-AuSPE) and then armed with Trp aptamer molecules (Apt-MWCNT-AuSPE). The prepared sensors were characterized using constant current-potentiometric stripping analysis (CC-PSA) and electrochemical impedance spectroscopy (EIS). The MWCNT-AuSPE and Apt-MWCNT-AuSPE were compared with respect to the linear detection range, limit of detection (LOD), accuracy, precision, repeatability. MWCNT-AuSPE and Apt-MWCNT-AuSPE demonstrate fast near-Nernstian response for PSA of Trp over the concentration ranging from 1.0 × 10(-9) to 2.0 × 10(-4) mol L(-1) and 1.0 × 10(-11) to 1.0 × 10(-4) mol L(-1) with detection limits of 3.6 × 10(-10) mol L(-1) and 4.9 × 10(-12) mol L(-1), respectively. Common interfering species present in the biological fluids (i.e., tyrosine, uric acid, ascorbic acid) showed no effects on the determination of Trp using CC-PSA. MWCNT-AuSPE and Apt-MWCNT-AuSPE represented well reproducibility and great precision with relative standard deviation (RSD) of 2.9% and 5.3% respectively. In comparison with the MWCNT-AuSPE, Apt-MWCNT-AuSPE provided higher sensitivity, selectivity and accuracy of Trp detection in real samples. Based on these findings, we propose the developed Apt-MWCNT-AuSPE as a simple detection method for analysis of Trp in biological samples.

Biomedical Probes Based on Inorganic Nanoparticles for Electrochemical and Optical Spectroscopy Applications

Inorganic nanoparticles usually provide novel and unique physical properties as their size approaches nanometer scale dimensions. The unique physical and optical properties of nanoparticles may lead to applications in a variety of areas, including biomedical detection. Therefore, current research is now increasingly focused on the use of the high surface-to-volume ratios of nanoparticles to fabricate superb chemical- or biosensors for various detection applications. This article highlights various kinds of inorganic nanoparticles, including metal nanoparticles, magnetic nanoparticles, nanocomposites, and semiconductor nanoparticles that can be perceived as useful materials for biomedical probes and points to the outstanding results arising from their use in such probes. The progress in the use of inorganic nanoparticle-based electrochemical, colorimetric and spectrophotometric detection in recent applications, especially bioanalysis, and the main functions of inorganic nanoparticles in detection are reviewed. The article begins with a conceptual discussion of nanoparticles according to types, followed by numerous applications to analytes including biomolecules, disease markers, and pharmaceutical substances. Most of the references cited herein, dating from 2010 to 2015, generally mention one or more of the following characteristics: a low detection limit, good signal amplification and simultaneous detection capabilities.

A carbon ion beam irradiated MWCNT/AuNPs composite sensor for a sensitive assay of purine-nucleosides of DNA

Sensor for purine nucleosides has been developed using irradiation with high energy carbon ion beam.

An amperometric sensor for detection of tryptophan based on a pristine multi-walled carbon nanotube/graphene oxide hybrid

A pMWCNT/GO hybrid shows excellent catalytic activity towards tryptophan oxidation and can thus be used as electrode material in a biosensor for sensing tryptophan sensitively and selectively.

High-density assembly of chemiluminescence functionalized gold nanodots on multiwalled carbon nanotubes and their application as biosensing platforms

A one-step strategy was developed for high-density assembly of N-(aminobutyl)-N-(ethylisoluminol) (ABEI) functionalized gold nanodots onto the sidewalls of chitosan-grafted multiwalled carbon nanotubes (cs-MWCNTs) via the reduction of HAuCl4 with ABEI in the presence of cs-MWCNTs, resulting in novel hybrid nanomaterials with excellent chemiluminescence and electrochemiluminescence properties.

A molecularly imprinted electrochemical enzymeless sensor based on functionalized gold nanoparticle decorated carbon nanotubes for methyl-parathion detection

Schematic of MP MIP sensor and the possible mechanism.