Analytical Methods (v.9, #1)

Front cover (1-2).

Contents list (3-9).

In this paper, the principles of capillary electrophoretic on-line preconcentration are introduced. The application of these techniques and their combinations in alkaloid analysis is reviewed. The trends of capillary electrophoretic on-line preconcentration in alkaloid analysis are discussed.

A benzoxazole functionalized fluorescent probe for selective Fe3+ detection and intracellular imaging in living cells by Zheng Yang; Xinxin Bai; Siyue Ma; Xiangrong Liu; Shunsheng Zhao; Zaiwen Yang (18-22).
A rhodamine based 2-aminobenzoxazole functionalized probe was presented for reversible detection of Fe3+ driven by the coordination of Fe3+ to 2-aminobenzoxazole that resulted in strong fluorescence emission and color change. Fluorescent imaging in living L929 and MG-63 cells offered a promising candidate for mapping Fe3+ in biological samples.

A simple and green method for the synthesis of fluorescent silver nanocluster–nanoparticle complexes (AgNCs–AgNPs) is presented. In this method, Ag+ was reduced to form AgNCs–AgNPs using hydrogen as the reductant and sodium polyacrylate as a soft template at room temperature. The AgNCs–AgNPs possess strong fluorescence, and the fluorescence property has high sensitivity and excellent selectivity to Ca2+. An approach based on the enhancement of fluorescence was firstly applied for the quantitative determination of Ca2+. The approach displays a wide linear response, excellent selectivity and high sensitivity, and provides a potential candidate for the quantitative determination of Ca2+ in biological systems.

Standoff chemical imaging finds evidence for Jackson Pollock's selective use of alkyd and oil binding media in a famous ‘drip’ painting by Kathryn A. Dooley; James Coddington; Jay Krueger; Damon M. Conover; Murray Loew; John K. Delaney (28-37).
Near-infrared diffuse reflectance imaging spectroscopy (NIR-RIS, 1000 to 2500 nm) was used to map the use of alkyd and oil paints in Jackson Pollock's Number 1, 1950 (Lavender Mist), one of his most important ‘drip’ or ‘poured’ paintings. Pollock's drip paintings were created by allowing the paint to “pour” from his brush/stick down onto the canvas. Prior analysis of micro-samples from drip paintings found Pollock was extensively using alkyd-resin paints by 1949 (in addition to oil) which led to a hypothesis that he selectively used alkyd and oil paints to obtain the fluid, gestural marks characteristic of his mature style. To determine where he used alkyd and oil paints in Lavender Mist, we utilized near-infrared spectral regions (1615–1850 and 1860–2200 nm) that allowed discrimination between these binding media, especially when the first derivative spectra were examined. The near-infrared image cubes were collected with a high-spectral resolution (2.8 nm sampling) hyperspectral camera. Using convex geometry-based multivariate analysis, three spectral endmembers were identified: oil, alkyd resin, and a third binder. The mapping results show oil binding medium was confined to the uppermost layers of the painting, corresponding with long, more-or-less straight, white skeins of paint. An alkyd binding medium maps to long white skeins of paint (both straight and curved), in addition to many irregular-shaped, disconnected regions of white and blue-green paint, indicating the mapped alkyd paint is below other layers. The third spectral endmember maps to an off-white paint that may be a mixture/layering of oil and alkyd media. X-Ray fluorescence imaging spectroscopy and site-specific reflectance spectroscopy (350–2500 nm) found evidence that the oil and alkyd white paints contain zinc and titanium whites. However, the oil paint likely contains the anatase mineral form of titanium dioxide, whereas the alkyd paint likely contains rutile titanium dioxide. As these pigments do not absorb near-infrared radiation, the mapping results demonstrate the ability to use NIR-RIS to map modern paint binders. Furthermore, the use of oil and alkyd paints seem to be intentional choices by Pollock: the oil because it has more body, and the alkyd because it is more fluid and dries more quickly.

Online fluorescence anisotropy immunoassay for monitoring insulin secretion from islets of Langerhans by Adrian M. Schrell; Nikita Mukhitov; Lian Yi; Joel E. Adablah; Joshua Menezes; Michael G. Roper (38-45).
Insulin secretion from islets of Langerhans is a dynamic process that is essential for maintaining glucose homeostasis. The ability to measure dynamic changes in insulin levels upon glucose stimulation from single islets will allow testing of therapeutics and investigating mechanisms of defective secretion observed in metabolic diseases. Most approaches to date for measurement of rapid changes in insulin levels rely on separations, making the assays difficult to translate to non-specialist laboratories. To enable rapid measurements of secretion dynamics from a single islet in a manner that will be more suitable for transfer to non-specialized laboratories, a microfluidic online fluorescence anisotropy immunoassay was developed. A single islet was housed inside a microfluidic chamber and stimulated with varying glucose levels from a gravity-based perfusion system. The total effluent of the islet chamber containing the islet secretions was mixed with gravity-driven solutions of insulin antibody and Cy5-labeled insulin. After mixing was complete, a linearly polarized 635 nm laser was used to excite the immunoassay mixture and the emission was split into parallel and perpendicular components for determination of anisotropy. Key factors for reproducible anisotropy measurements, including temperature homogeneity and flow rate stability were optimized, which resulted in a 4 nM limit of detection for insulin with <1% RSD of anisotropy values. The capability of this system for measuring insulin secretion from single islets was shown by stimulating an islet with varying glucose levels. As the entire analysis is performed optically, this system should be readily transferable to other laboratories.

Checkpoints for preliminary identification of small molecules found enriched in autophagosomes and activated mast cell secretions analyzed by comparative UPLC/MSe by Chad P. Satori; Marzieh Ramezani; Joseph S. Koopmeiners; Audrey F. Meyer; Jose A. Rodriguez-Navarro; Michelle M. Kuhns; Thane H. Taylor; Christy L. Haynes; Joseph J. Dalluge; Edgar A. Arriaga (46-54).
We report the use of ultra high performance liquid chromatography (UPLC) coupled with acquisition of low- and high-collision energy mass spectra (MSe) to explore small molecule compositions that are unique to either enriched-autophagosomes or secretions of chemically activated murine mast cells. Starting with thousands of features, each defined by a chromatographic retention time, m/z value and ion intensity, manual examination of the extracted ion chromatograms (XIC) of chemometrically selected features was essential to eliminate false positives, occurring at rates of 33, 14 and 37% in samples of three biological systems. Forty-six percent of features that passed the XIC-based checkpoint, had IDs in compound databases used here. From these, 19% of IDs had experimental high-collision energy MSe spectra that were in agreement with in silico fragmentation. The importance of this second checkpoint was highlighted through validation with selected commercially available standards. This work illustrates that checkpoints in data processing are essential to ascertain reliability of unbiased metabolomic studies, thereby reducing the risk of generating ‘false identifications’ which is a major concern as ‘omics’ data continue to proliferate and be used as platforms to launch novel biological hypotheses.

Highly sensitive poisoning-resistant optical carbon dioxide sensors for environmental monitoring by Eva Fritzsche; Pia Gruber; Susanne Schutting; Jan P. Fischer; Martin Strobl; Jens D. Müller; Sergey M. Borisov; Ingo Klimant (55-65).
A new optical carbon dioxide sensor for environmental monitoring is presented. It combines a robust and long-term stable sensing material with a compact read-out device. The sensing material relies on a NIR pH indicator immobilized into ethyl cellulose along with a quaternary ammonium base. The perfluorinated polymer Hyflon AD 60 used as a protection layer significantly enhances the long-term and mechanical stability of the sensor foils, as well as the robustness against poisoning gases, e.g. hydrogen sulfide. The sensor can be stored under ambient conditions for more than six weeks, whereas sensors covered with silicone rubber deteriorate within one week under the same conditions. The complete sensor device is applicable after a three-point (re)calibration without a preconditioning step. The carbon dioxide production and consumption of the water plant Egeria densa was measured in the laboratory. Furthermore, results of profiling carbon dioxide measurements during a research cruise on the Baltic Sea at water depths up to 225 m are presented.

A balance-in-a-box: an integrated paper-based weighing balance for infant birth weight determination by Devi D. Liana; Burkhard Raguse; J. Justin Gooding; Edith Chow (66-75).
The birth weight of babies is generally used as a health state indicator but is not often recorded in resource-poor settings due to limited access to medical clinics. In this work we develop a low-cost and lightweight “balance-in-a-box” that could allow remote communities to have access to a weighing balance. The “balance-in-a-box” is a cardboard mailing box which is reconstructed to form the balance tray, with a novel piezoresistive pressure sensor underneath, a paper-based readout system and battery integrated with copper tape to form an electrical circuit. The readout system comprises multiple segments of electrochromic Prussian blue/polyaniline on conductive gold nanoparticle films connected by graphite resistive separators. The color of each segment is voltage dependent and is directly correlated to the weight applied to the pressure sensor. It is shown that by varying the properties of the readout system (total resistance, individual resistance values of the graphite separators and number of gold nanoparticle segments) that it is possible to tune the critical weight values in which the gold nanoparticle segments change color. Overall, this work demonstrates that babies can be classified as very low birth weight (<1.5 kg), low birth weight (1.5 to 2.5 kg) or healthy (>2.5 kg) using a simple, integrated paper-based system that can be deployed to resource-poor settings where babies are at greatest risk of being underweight and requiring health care treatment.

From among a number of matrices, tissues are the most complex and difficult to prepare for the determination of analytes. Trace amounts of numerous substances in tissues must be determined with adequate precision and accuracy. The QuEChERS technique was used for the extraction of six PAHs from four kinds of tissues (porcine, avian, cod, and herring). HPLC coupled with a fluorescence detector and GC/MS were used for the final analysis. In each tissue, moisture and fat content were determined. The method recovery rate was 84–101% (SD = 0.06–0.12) for the pork tissue, 88–107% (SD = 2.6–6.9) for the cormorant tissue, and R = 89–102% (SD = 4.9–8.9) for the cod tissue. The HPLC/FLD determination of pyrene in the herring tissue was the most problematic. Pyrene was determined by GC/MS. The recovery was 93% (SD = 5.5). For three tissues (i.e. pork, cormorant, and fish), homogeneity and the certified values were determined. The above-mentioned tissues were candidates for new certified reference materials.

In this paper, a highly defective mesoporous carbon (DMC) and room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM·PF6) were applied to fabricate a novel carbon paste electrode for the electrochemical sensing of rutin. The electrochemical properties of rutin on the modified electrode (IL/DMC/PE) were carefully investigated by cyclic voltammetry, electrochemical impedance spectroscopy and chronocoulometry. Electrochemical parameters of rutin on the surface of the modified electrode were determined with the electron transfer coefficient (α) as 0.53, the electron transfer number (n) as 2, the heterogeneous electron transfer rate constant (ks) as 37.1 s−1, the diffusion coefficient as 1.34 × 10−4 cm2 s−1 and saturated absorption capacity as 10.9 nmol cm−2. Under the optimal conditions, rutin could be detected in the concentration range from 0.008 μmol L−1 to 4.0 μmol L−1 with the detection limit as 1.17 nmol L−1 (S/N = 3) by square-wave voltammetry. The proposed sensor was further applied successfully for the determination of rutin content in some real samples, including Ruta graveolens extract, pharmaceutical tablets and orange juice.

This paper presents a comprehensive analysis of artistic paints produced in the 19th century by a French art materials supplier Richard Ainès. Improved mild extraction with hydrofluoric acid enabled the observation of intact organic dyes. Reversed-phase liquid chromatography with diode-array and mass spectrometry detection was utilised for the identification of 35 dyes present in yellow and red paint samples, and in selected plant extracts. The developed analytical method allowed more efficient separation of several isomeric flavonoid and anthraquinone dye components of the paint samples. Persian berries and weld were identified as the dye sources in the yellow paint samples. The red oil paint had been coloured with the madder lake-type plant. Studies of dye extracts of the historical samples show the presence of uncommon dye components (quercetin-O-rhamnoside-glucuronide and rhamnasin-O-rhamnoside-glucoside) that could enhance this identification. The paint samples were additionally analyzed by X-ray fluorescence (XRF) and scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX). Aluminum, Sn, Zn, Ca, Cu, S, Si, and K were detected confirming the presence of aluminium hydrate and tin salts as carriers, as well as chalk, and other components used during the production of these paints. The SEM with the BSE detector images revealed the homogenous texture of finely ground lake pigments.

Rapid determination of the Mn average oxidation state of Mn oxides with a novel two-step colorimetric method by Yanhua Zhu; Xinran Liang; Huaiyan Zhao; Hui Yin; Mingming Liu; Fan Liu; Xionghan Feng (103-109).
The Mn average oxidation state (Mn AOS) of Mn oxides has a significant impact on their reactivity towards trace metals and organic contaminants via sorption, catalysis and oxidation processes. Accurate determination of the Mn AOS is a key step to understanding the structures, composition, physicochemical properties, environmental behaviors and potential applications of Mn oxides. Here, a rapid two-step colorimetric method was developed to determine the Mn AOS of various Mn oxides, and it was tested on several Mn oxides (vernadite, acid birnessite and hausmannite). We also determined the Mn AOS of these Mn oxides with the conventional oxalic acid–permanganate back titration method and X-ray absorption near-edge spectroscopy (XANES) for comparison. In this rapid two-step colorimetric method, leucoberbelin blue I (LBB) and formaldoxime colorimetry were employed to obtain the oxidation numbers of high-valent Mn and total Mn, respectively, which were then used to calculate the Mn AOS. The colorimetric measurements are of considerable color stability and high sensitivity, thus enabling rapid, convenient and highly accurate determination of the Mn AOS compared with conventional methods. In addition, the required sample amount is greatly reduced (from ∼0.05 g to ∼0.005 g), making the proposed method an appropriate strategy for micro-volume samples.

Soluble lipoprotein receptor-related protein immunoreactive species in cell culture media and serum replacement supplements by Chantel Fitzsimmons; Daniel Johnstone; Katherine Conant; Coryse St Hillaire; Carl H. Parsons; Monique Stins; Robert Moir; Elizabeth A. Milward (110-116).
The low-density lipoprotein receptor-related protein (LRP) is a large multifunctional cell surface membrane receptor capable of binding over 50 ligands. These include molecules important in Alzheimer's disease such as the amyloid β-protein precursor (AβPP), the β-amyloid (Aβ) peptide and apolipoprotein E (ApoE). Full length LRP consists of a 515 kDa extracellular ligand binding α-chain and an 85 kDa membrane spanning β-chain. A soluble form of LRP (sLRP) present in human plasma retains the ability to bind ligands, including Aβ. This soluble form is an ectodomain fragment generated from the membrane bound form of the receptor by proteolytic cleavage. Here we report data demonstrating that some commercial ‘serum-free’ supplements and ‘serum-free’ media contain unlisted sLRP immunoreactive species that may reflect the presence of undefined serum protein extracts in these ‘serum-free’ preparations. This has the potential to interfere with experimental results and interpretation in a range of cell culture studies involving LRP or any of its ligands and possibly also other serum proteins.

Herein, a new electrochemical sensing protocol was designed for ultrasensitive detection of copper(ii) (Cu2+) at the attomolar level based on target-induced click conjugation using horseradish peroxidase (HRP) as an indicator and gold nanoparticle (AuNP) as an enhancer. The click conjugation was carried out between the immobilized azido-DNA on the electrode and alkynyl-DNA/HRP-labeled AuNP. With the help of sodium ascorbate, target Cu2+ ion was initially reduced into Cu+, and the as-produced Cu+ could catalyze the azide–alkyne click reaction. Accompanying gold nanoparticles, the carried HRP molecules could electrochemically reduce the substrate (H2O2); this resulted in a strong electronic signal. The digital readout was relative to the amount of the peroxidase and indirectly reflected the Cu2+ concentration in the testing media. Under optimal conditions, the as-prepared sensor exhibited good electrochemical response towards Cu2+ in the 1.0 aM to 10 μM dynamic working range with a 0.38 aM detection limit. This methodology also displayed a high selectivity for Cu2+ relative to other potentially interfering ions due to the specific Cu+-induced azide–alkyne click reaction, and was applicable for monitoring Cu2+ in real river samples. Our strategy has a good potential for use in environmental surveys.

A novel ditopic chemosensor for cadmium and fluoride and its possible application as a pH sensor by Additi Roy Chowdhury; Pritam Ghosh; Suparna Paul; Samuzal Bhuyan; Jagadeesh C. Bose K; Sudit Mukhopadhyay; Priyabrata Banerjee (124-133).
A urea-based BPC [1,5-bis(perfluorophenyl)carbonohydrazide] molecule with four acidic NH protons has been synthesized by a facile synthetic process. The molecule was found to be a ditopic chemosensor for Cd2+ and F ions. BPC was synthesized from low-cost starting materials, dinitrophenyl hydrazine and triphosgene. The host–guest interactions between the ions (Cd2+ and F) were not only confirmed by convenient spectroscopic techniques such as UV-Vis, PL, 1H-NMR, FT-IR, and cyclic voltammetry but also through modern DFT, the results of which were in good agreement with the experimental results. In vitro studies in human cancer cells (HeLa cells) were successfully performed with BPC and Cd2+ using fluorescence microscopy. The reversible UV-Vis response for BPC with F, OH and H+ mimics multiple logic functions and can be used for several complex electronic circuits based on logic operations. The pH sensor (BPC) can be further interfaced with suitable circuitry interfaced with appropriate programming for ease of access and enhancement of its practical applications.

Bisphenol A (BPA) is an important and widely used industrial chemical that is harmful to both the environment and human health. Graphene/platinum nanoparticles (Gr/PtNPs) functionalized with heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin (TM-β-CD) were successfully synthesized using a simple wet chemistry strategy. The new nanocomposite had both the excellent properties of the Gr/PtNPs (large surface area, high conductivity and excellent electrocatalytic activity) and the cyclodextrin (host–guest supramolecular recognition and enrichment capability). It had a remarkable synergistic effect on the electrochemical reaction of BPA and a highly sensitive electrochemical sensor was developed to detect BPA. The oxidation peak current of BPA was greatly enhanced at the TM-β-CD-Gr/PtNPs modified glassy carbon electrode (GCE) compared with the Gr/PtNPs/GCE and the TM-β-CD-Gr/GCE. Under the optimum condition, the peak current for BPA increased linearly with concentration in the range 5.0 × 10−8 to 8.0 × 10−5 mol L−1. The detection limit was 1.5 × 10−8 mol L−1 (S/N = 3). This sensor is cost-effective and convenient and shows great potential for the detection of trace amounts of phenol compounds in environmental pollutants.

A minimalist Chinese liquor identification system based on a colorimetric sensor array with multiple applications by Li Jia-wei; Hou Chang-jun; Huo Dan-qun; Yang Mei; Zhang Su-yi; Ma Yi; Lin Yang (141-148).
The components of Chinese liquor are influenced by geographical origins and raw materials. Currently, Chinese liquor identification systems are limited and have a single application that can distinguish only one nature of flavor type and production area. Four chemical reagents were successfully used to build this 3 × 1 simple colorimetric sensor array. This is the first time that a highly accurate colorimetric Chinese liquor sensor array was fabricated using only a few array points. It could distinguish four types of Chinese base liquors from Luzhou Laojiao, nine types of Chinese liquors with different flavor types, and seven types of Chinese liquors from different geographical origins. In order to ensure the accuracy, seven parallel samples were analyzed with water used as a control. PCA proved this sensor array to be effective. In LDA and HCA, every group was clearly classified. The “leave-one-out” cross-validation method was applied with 100% prediction ability in the three experiments. According to the results of the PCA, HCA, LDA, and the “leave-one-out” method, we can deduce that this colorimetric sensor array with multiple applications has great performance in discriminating Chinese liquors.

Soft silicones are commonly used as a skin-friendly adhesive in wound care products. To prevent infection, silver (Ag) is frequently incorporated into wound dressings as an antimicrobial agent. Recently, wound dressings with Ag directly incorporated into soft silicone wound/skin contact layers have become commercially available. Thus there is a need for accurate Ag determination in these dressings. In this work the determination of Ag in soft silicones by inductively coupled plasma optical emission spectroscopy is described. This method utilises dodecylbenzene sulfonic acid for the digestion of silicones followed by extraction of Ag with hydrochloric acid (HCl). This method shows excellent accuracy (99–104%) and precision (0.3–4.4% CV) by combining matrix matched standards and internal standardization with indium. A short analysis time of 3 minutes per sample is required and the limit of quantification is 27 μg, corresponding to 540 mg kg−1 for a typical sample weight of 50 mg, which is sufficient to determine the Ag content in commercially available soft silicone wound dressings. The procedure is carried out as an open vessel digestion using a single polypropylene tube which minimizes the need for additional equipment and preparative work.

A label-free SERRS-based nanosensor for ultrasensitive detection of mercury ions in drinking water and wastewater effluent by Dan Song; Rong Yang; Haoyu Wang; Wei Li; Hongchen Wang; Hui Long; Feng Long (154-162).
Precisely probing mercury ions (Hg2+) is of essential importance to human health and environmental protection. Although numerous methods have been developed to detect Hg2+ in water, challenges still remain for rapid, accurate, and reliable detection of Hg2+ at the ppt level. In this study, a novel label-free Fe3O4@Ag based surface-enhanced Raman resonance scattering (SERRS) nanosensor has been developed for ultrasensitive and highly selective detection of Hg2+. The detection mechanism is on the basis of competitive binding interaction of Hg2+ and malachite green (MG) with nano-sliver on Fe3O4@Ag magnetic beads (MBs). In the absence of Hg2+, the Raman signal intensity of MG is significantly enhanced by the nano-silver when MG adsorbs on the nano-silver surface via one nitrogen atom. In the presence of Hg2+, the redox reaction occurring between zero-valent nano-silver and Hg2+ leads to the formation of a Ag/Hg amalgam at the Fe3O4@Ag surface, which prevents the adsorption of MG on the nano-silver surface. Thereby, the SERRS signal intensity of MG proportionally decreased with increasing Hg2+ concentration. Under optimized conditions, the proposed label-free nanosensor showed unprecedented Hg2+ detection sensitivity of 10 pM (2 ppt) with excellent selectivity. This simple sensor system can be directly applied for an inexpensive and easy-to-use monitoring method of Hg2+ in drinking water and wastewater treatment effluents that contains complicated organic and inorganic interferents.

Ni doped Ag@C core–shell nanomaterials and their application in electrochemical H2O2 sensing by Qinglin Sheng; Yu Shen; Jian Zhang; Jianbin Zheng (163-169).
In this work, attractive core–shell structured nanomaterials consisting of Ni doped Ag@C were synthesized. Further, a hydrogen peroxide (H2O2) sensor was fabricated by modifying the Ni doped Ag@C (Ni/Ag@C) nanocomposites onto the surface of a glassy carbon electrode (GCE). The composition and morphology of the Ni/Ag@C nanocomposites were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy. The results indicated that the nanocomposites were synthesized successfully and showed similar grain sizes and favorable dispersity. The electrochemical and electrocatalytic properties of the Ni/Ag@C nanocomposites were studied by cyclic voltammetry. An electrochemical investigation showed that the Ni/Ag@C nanocomposite modified GCE exhibited a good electrocatalytic ability for H2O2 reduction and it was used for the determination of H2O2. The linear range for H2O2 determination was from 0.03 mM to 17.0 mM with a detection limit of 0.01 mM (S/N = 3). The sensitivity was 22.94 μA mM−1 cm−2. It is expected that the application of the Ni/Ag@C nanocomposites could be extended to the construction of other sensors and applications in various sensing fields.

Photocatalytic air-purification: a low-cost, real-time gas detection method by Donal A. Keane; Niki Hamilton; Lorraine T. Gibson; Suresh. C. Pillai; Justin D. Holmes; Michael A. Morris (170-175).
This research demonstrates the use of a gas detector as a feasible alternative to the standardized analytical methods typically found in photocatalytic air-purification ISO standard tests and academic literature. A methyl mercaptan detector is calibrated and validated (for linearity) using a standard gas generator. The detector can be directly connected to the photoreactor exit allowing real-time span gas measurement with data-logging at one minute intervals. The detector successfully differentiated samples with different photocatalytic performance. The use of such detectors offers an easy-to-use, low-cost alternative to gas measurement with applications in academic research, proof-of-concept photocatalytic tests and also as an educational tool.

Back cover (177-178).