Analytical Methods (v.11, #35)

Front cover (4393-4393).

Inside front cover (4394-4394).

Contents list (4395-4400).

Curcumin (CM) is a natural pigment with a crystalline structure and is the most important curcuminoid in turmeric. CM is effective in treating a wide range of diseases, especially cancers, owing to its antioxidant, anti-proliferative, antibacterial and anti-inflammatory properties. Electrochemical techniques are powerful and widely used analytical methods that provide valuable information about electrochemical systems. Their dedicated performance, as well as their ability to provide very low detection limits, are the most important benefits of these methods. Electrochemical sensors have been used as good tools for the detection of CM owing to their simplicity, accuracy, high sensitivity and selectivity, and reasonable price. However, owing to the poor response of this compound, it is difficult to detect it directly at the surfaces of bare electrodes. Various modifiers have been used to overcome this problem and to increase the sensitivity and selectivity of CM detection sensors. In this research, various types of bare and modified electrodes used in the detection of CM are briefly investigated and their performance is compared scrupulously.

An electrochemical immunosensing system on patterned electrodes for immunoglobulin E detection by Seung Jun Oh; Jun Ki Ahn; Hun Park; Yesol Song; Seong Jung Kwon; Hang-Beum Shin (4410-4415).
We present an electrochemical biosensing platform which enables immunoglobulin E (IgE) detection from eight independent biological samples. The performance of the electrochemical immunosensor was comparable to the conventional optical assay. This biosensor would be a powerful tool for multiple IgE diagnoses, taking advantage of outstanding electrochemical assays.

Physical adsorption techniques used for the detection of Hg2+ have characteristic disadvantages that compromise the Hg2+ sensitivity, and thus present false signals that affect the resulting outcome. Here, in order to avoid the aforementioned drawbacks and improve detection sensitivity, a single-stranded DNA sequence modified with an amino group and FAM (carboxyfluorescein) was immobilized onto the surface of graphene oxide (GO). A co-anchor strand with an amino group was then attached to further develop the sensor, which resulted in an improved efficiency of DNA sequence immobilization onto the GO surface and an improvement in the limit of detection of Hg2+. The detection limit was found to be 2.52 nM in this study.

In this work, sulfur-doped graphene quantum dots (S-GQDs) with stable green fluorescence were successfully synthesized by hydrothermal pyrolysis in the presence of glucose and mercaptosuccinic acid as the carbon and sulfur sources, respectively, for rapid and sensitive detection of hemoglobin (Hb). The as-prepared S-GQDs have a uniform size with a mean particle diameter of 4.5 ± 0.5 nm and an excellent quantum yield of 71%. The high sulfur content of 3.7% in the S-GQD network can not only increase the quantum yield of S-GQDs but also enhance the electron density of S-GQDs for effective Hb detection through π–π interactions. The S-GQDs exhibit an excellent sensitivity toward Hb detection and a linear range of 10–1000 nM with a low limit of detection of 0.28 and 0.48 nM in phosphate buffer solution and human serum, respectively, is observed. Moreover, the fluorescence response of S-GQDs exhibits high selectivity toward Hb over 16 other interferents including metal ions, amino acids, and proteins. The results in this work clearly demonstrate the easy and facile fabrication of S-GQDs for sensitive and selective detection of biomolecules without surface modification, which can open a new way to develop highly efficient and robust sensing probes for the detection of biomarkers, metal ions and organic metabolites in biological applications.

Discrimination of papers used in conservation and restoration by the means of the voltammetry of immobilized microparticles technique by Francesca Di Turo; Carolina Mai; Andrea Haba-Martínez; Antonio Doménech-Carbó (4431-4439).
Herein, voltammetry of immobilized microparticles (VIMP) is employed for the discrimination of papers used in the conservation and restoration field. The electrochemical parameters are used for the discrimination of different samples coming from China, Japan and Korea. The method is based on the recording of the voltammetric response of microparticulate deposits from acetone extracts of paper samples deposited on a glassy carbon electrode in contact with 1.0 M H2SO4. The voltammetric responses attributed to the lignin and the oxidation products of the cellulose were collected and are discussed. The grouping of samples was obtained upon the application of the chemometric methods, as well as the ratio between the prominent voltammetric peaks. In the first case, chemometrics allows identification of the different papermaker production techniques amongst the samples, while the experimental data permits the grouping of the papers according to their internal chemical differences. The proposed method aims at help the archaeometric study of papers using a micro-invasive method, which allows the chemistry and the manufacturing methods of ancient papers to be distinguished.

Quinolinyl-azo-naphthol (HL) is a selective turn-on chemosensor for Al3+ in the presence of other ions and the excitation at visible light (537 nm) shows a 750 fold enhancement of emission at 612 nm. The limit of detection (LOD) is 0.69 nM (3σ method), which is far below the WHO recommended value for Al3+ in drinking water (7.41 μM). The composition of the isolated complex [AlL2]+ is supported by ESI-MS spectral data and Job's plot. Upon addition of aqueous F to the [AlL2]+ solution the emission intensity is quenched and the LOD of F is 0.63 nM. HL is thus instrumental for the design of portable kits for the detection of fluoride contamination in drinking water. The probe undergoes azo-hydrazo tautomerization and also exhibits an INHIBIT logic gate with Al3+ and F as chemical inputs by monitoring the emission mode at 612 nm.

Development of an endoplasmic reticulum-targeting fluorescent probe for the two-photon imaging of hypochlorous acid (HClO) in living cells by Wenhui Song; Baoli Dong; Yaru Lu; Xiuqi Kong; Abdul Hadi Mehmood; Weiying Lin (4450-4455).
As an important reactive oxygen species, hypochlorous acid (HClO) could irreversibly destroy Ca2+-ATPase to inhibit Ca2+ transport in the endoplasmic reticulum (ER), which may eventually lead to cell dysfunction and cell death. Herein, we present a novel ER-targeting fluorescent probe (NHS-ER) for the two-photon detection of HClO in living cells. NHS-ER was constructed using naphthalimide and showed strong green fluorescence. In response to HClO, the fluorescence intensity at 502 nm gradually decreased, and the probe showed excellent selectivity to HClO over other biologically relevant ROS. Moreover, the probe NHS-ER showed ER-targeting properties and could be successfully applied to the two-photon imaging of HClO in living cells.

Detection of Fe3+ using a novel hyperbranched polymeric spectral sensor by Wenbo Guo; Tengxuan Tang; Shenzhou Lu; Dongmei Xu (4456-4463).
Fe3+ was efficiently detected using a novel turn-on hyperbranched polymeric spectral sensor based on the linear relationship between the absorbance at 562 nm (or the fluorescence intensity at 578 nm) of the sensor–Fe3+ solution and the Fe3+ concentration, as well as the visible (from colorless to pink) and fluorescence (from dark to orange-red) color changes. The sensor showed great selectivity to Fe3+ and the competition ions had negligible influence. Further, the sensor could be applied in real water sample assays with a similar accuracy to that of AAS and it could be facilely recovered using Na2S.

A portable, disposable, and low-cost optode for sulphide and thiol detection by Giancarla Alberti; Valeria M. Nurchi; Lisa R. Magnaghi; Raffaela Biesuz (4464-4470).
In this study, a portable, disposable, and low-cost ionophore-based optical sensor is presented for the monitoring of sulphides and thiols. This sensor was obtained by fixing a classical dye, 5,5′-dithiobis(2-nitrobenzoic acid) (Ellman's reagent, ELL), on an unusual solid support. Indeed, we decided to anchor the dye on the commercial paper sheet known as the “Colour Catcher®” (herein, named under the acronym CC), commonly used in the washing machine to prevent colour run problems. The device obtained can be regarded as an optical sensor since the indicator dye fixed on the solid material changes its spectral properties (colour and hence the UV-vis spectrum) upon contact with the analyte. The relationship between the analyte content and changes in the UV-vis spectrum of the sensor has been provided using partial least squares regression (PLS).

Improved measurement of the calorific value of pulverized coal particle flow by laser-induced breakdown spectroscopy (LIBS) by Wenbing Li; Meirong Dong; Shengzi Lu; Shishi Li; Liping Wei; Jianwei Huang; Jidong Lu (4471-4480).
The real-time quantitative analysis of the calorific value of pulverized coal particle flow is important for the efficient and clean combustion of coal. Herein, a piezoelectric vibratory feeder produced a continuous flow of pulverized coal particles, which were detected by laser-induced breakdown spectroscopy (LIBS), and the corresponding spectral information was obtained. The obtained data matrices, after removing abnormal data, were subjected to normalization in a row-wise manner to improve the signal repeatability. Then, the second convolution derivative was conducted on the LIBS spectra to correct the spectral interference. To acquire appropriate number and type of variables for the PLS quantitative model, the variable selection methods of genetic algorithm (GA) and synergy interval partial least squares (siPLS) in the quantitative model were analyzed and compared. The results showed that spectral correction could be used to improve the predictive precision and accuracy of the quantitative model based on the full spectrum. The predictive accuracy of the model with variables selected by siPLS was further improved. The minimum average relative error (RE) of the predictive values for the validation samples was 1.53%, and the average error of the proposed model for the calorific value quantitative analysis was 0.22 MJ kg−1. The measurement indicated that LIBS could realize an accurate and precise real-time quantitative analysis of the pulverized coal particle flow.

Scalable calibration transfer without standards via dynamic time warping for near-infrared spectroscopy by Congming Zou; Huimin Zhu; Junru Shen; Yue He; Jiaen Su; Xiaqiong Fan; Hongmei Lu; Zhimin Zhang; Yi Chen (4481-4493).
Calibration transfer is crucial for near-infrared spectroscopy (NIR) to avoid time-consuming and labor-intensive recalibration. Classical methods require standard samples from both the master and slave spectrometers, which is the major bottleneck of large-scale NIR applications. In this study, the calibration transfer method based on variable penalty dynamic time warping (CT-VPdtw) is proposed, which can handle standard-free calibration transfer well and greatly expand the application ranges of the established model. The corn dataset with standards from Cargill and the standard-free wheat dataset from international diffuse reflectance conference (IDRC) 2016 are used to benchmark the transfer ability. When transferring from M5 to MP5 of the corn dataset, the RMSEP reduced from 0.86 to 0.152 by CT-VPdtw, 0.176 by piecewise direct standardization (PDS) and 0.207 by domain-invariant partial-least-squares (di-PLS) respectively. For the standard-free wheat dataset, CT-VPdtw has significant advantages when comparing with PDS and di-PLS. Moreover, 11 slave spectrometers can be transferred to the master spectrometer with satisfactory RMSEPs easily in the wheat dataset, which demonstrates that CT-VPdtw has potential to perform large-scale calibration transfer. With the good transfer ability, the standard-free and large-scale advantages, CT-VPdtw has the potential to be a widely used method for calibration transfer in near-infrared spectroscopy. It was implemented and is available at https://github.com/HMzhu/CTVPdtw.

Zearalenone (ZEA) is a mycotoxin produced by a range of Fusarium fungi that infect cereals. ZEA may accumulate in cereals before the time of harvest. This paper describes the electrochemical behavior of zearalenone (ZEA) at a single-walled carbon nanotube screen-printed electrode (SWCNT-SPCE) using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). A single irreversible oxidation peak was observed. The DPV peak current of ZEA at a SWCNT-SPCE was noticeably enhanced due to ZEA adsorption on the electrode surface. Interestingly, the electrode showed no surface fouling by the oxidation products of ZEA and the repeated use of the electrode is feasible. The electrodeposited film was examined in an Fe(CN)63−/4− redox probe using CV and EIS measurements. A differential pulse adsorptive stripping voltammetric (DPASV) method for the determination of ZEA was developed. Under the optimized conditions, the anodic peak current of ZEA varies linearly with the ZEA concentration in the range 2.5 × 10−8− 1.0 × 10−6 M with a detection limit (LOD) of 5.0 × 10−9 M. The method was applied for the quantitative analysis of ZEA in cornflake samples.

MIP-coated Eu(BTC) for the fluorometric determination of lincomycin in eggs by Pu Wu; Qiuzheng Du; Yiyang Sun; Zhonghong Li; Hua He (4501-4510).
Lincomycin (LCM) has low ultraviolet (UV) absorption and thus, its determination requires complex instrumentation. Herein, a novel fluorometric probe based on an europium metal–organic framework coated with molecularly imprinted polymers (Eu(BTC)-MIP) was fabricated for the selective determination of LCM. First, highly luminescent Eu(BTC) was synthesized at room temperature. Then, through a simple co-polymerization process, MIPs were formed on the Eu(BTC) surface in the presence of LCM as template molecules to obtain Eu(BTC)-MIP. Eu(BTC) acted as a supporting substrate and antenna, and the MIP layer provided specific binding sites for LCM. Due to the advantages of both Eu(BTC) and MIPs, the probe had a long emission lifetime, strong fluorescence and specific recognition. Under the optimized conditions, the probe achieved the selective determination of LCM in the concentration range of 10 to 100 μg L−1 with a detection limit (LOD) of 7.18 μg L−1 (S/N = 3). This method is attractive because of its low cost, good selectivity, ready availability and simple manipulation without the need for complex instrumentation. Furthermore, the probe was successfully employed to detect LCM in egg samples with a recovery in the range of 88.02% to 97.29% and a relative standard deviation (RSD) of less than 3.46%. The satisfactory results demonstrate that the proposed probe based on Eu(BTC)-MIP can be used for the quantitative analysis of LCM and exhibits great potential for LCM determination in food samples. Also this study provides a new perspective for the determination of weak ultraviolet absorption or non-fluorescent substances without conjugated groups.

A matrix of perovskite micro-seeds and polypyrrole nanotubes tethered laccase/graphite biosensor for sensitive quantification of 2,4-dichlorophenol in wastewater by Shivamurthy Ravindra Yashas; Shadakshari Sandeep; Ballagere Puttaraju Shivakumar; Ningappa Kumar Swamy (4511-4519).
The present study demonstrates the fabrication of a laccase (lac) biosensor to detect and quantify 2,4-dichlorophenol (2,4-DCP) using a lac immobilized polypyrrole nanotube (PPyNT) and strontium copper oxide (SrCuO2) micro-seed composite modified graphite electrode. The SrCuO2 was synthesized using the co-precipitation method and PPyNT was grown using a methyl orange template. Scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction spectroscopy (XRD) techniques were used in the characterization of SrCuO2 and PPyNT. Furthermore, PPyNT and SrCuO2 were deposited onto the graphite electrode (Gr) and used as a platform for immobilization of the lac enzyme. SEM micrographs of the Gr/PPyNT/SrCuO2/lac electrode authenticated the deposition of nanomaterials and the enzyme, whereas the electrochemical impedance spectroscopy (EIS) results demonstrated the conducting nature of SrCuO2 and the successful enzyme immobilization. Compared to the unmodified electrode, Gr/PPyNT/SrCuO2/lac displayed a higher electrocatalytic behavior towards 2,4-DCP. The performance of the sensor was calibrated and evaluated using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The sensor exhibited a wide linear range (1–50 μM), a low detection limit (LOD) (0.18 μM) and high sensitivity (4.715 μA μM−1 cm−2). In addition, the proposed sensor exhibited good repeatability, reproducibility and stability. The real sample analysis results indicated an acceptable recovery up to 94%, which suggested that the sensor could be used to monitor 2,4-DCP.

Rapid extraction of free fatty acids from edible oil after accelerated storage based on amino-modified magnetic silica nanospheres by Qi Zhao; Jian Li; Yang Xu; Dandan Lv; Kanyasiri Rakariyatham; Dayong Zhou (4520-4527).
In this study, magnetic amino-modified silica nanospheres (Fe3O4@SiO2@NH2) were prepared via coating 3-aminopropyltriethoxysilane on the surface of Fe3O4 magnetite and used for the extraction of free fatty acids (FFAs) from edible oil samples for the first time. Fourier transform infrared spectroscopy and vibrating sample magnetometer analysis indicated that Fe3O4@SiO2@NH2 was successfully synthesized and exhibited superparamagnetism. The static and dynamic adsorption experiments showed that Fe3O4@SiO2@NH2 exhibited rapid adsorption equilibrium and satisfactory adsorption capacity towards FFAs. Under optimized adsorption and analytical conditions, Fe3O4@SiO2@NH2 could effectively extract FFAs from edible oil samples with excellent linearities (0.2–1000 nmol g−1), extremely low limits of detection (0.09–0.46 nmol g−1), and satisfactory recoveries (77.4–101.8%) and precisions (3.1–9.6%). Moreover, compared to classic solid phase extraction using a commercial cartridge, the current magnetic solid phase microextraction method has many advantages including easier preparation of adsorbents, a simpler sample handling procedure and lower cost. Therefore, this method provides a promising alternative to the traditional method for the analysis of FFAs to ensure the sanitation and quality of oils.

Droplet incubation and splitting in open microfluidic channels by Samuel B. Berry; Jing J. Lee; Jean Berthier; Erwin Berthier; Ashleigh B. Theberge (4528-4536).
Droplet-based microfluidics enables compartmentalization and controlled manipulation of small volumes. Open microfluidics provides increased accessibility, adaptability, and ease of manufacturing compared to closed microfluidic platforms. Here, we begin to build a toolbox for the emerging field of open channel droplet-based microfluidics, combining the ease of use associated with open microfluidic platforms with the benefits of compartmentalization afforded by droplet-based microfluidics. We develop fundamental microfluidic features to control droplets flowing in an immiscible carrier fluid within open microfluidic systems. Our systems use capillary flow to move droplets and carrier fluid through open channels and are easily fabricated through 3D printing, micromilling, or injection molding; further, droplet generation can be accomplished by simply pipetting an aqueous droplet into an empty open channel. We demonstrate on-chip incubation of multiple droplets within an open channel and subsequent transport (using an immiscible carrier phase) for downstream experimentation. We also present a method for tunable droplet splitting in open channels driven by capillary flow. Additional future applications of our toolbox for droplet manipulation in open channels include cell culture and analysis, on-chip microscale reactions, and reagent delivery.

Back cover (4537-4538).