Analytical Methods (v.10, #32)

Front cover (3899-3899).

Inside front cover (3900-3900).

Contents list (3901-3905).

Surface plasmon resonance (SPR) technology has widened the use of sophisticated methodologies for studying biological and chemical systems in terms of analyzing protein structures, determining the analyte concentration and retrieval of kinetic and equilibrium parameters. However, one of the main challenges in the use of SPR sensors is to detect biological or chemical entities of low molecular weight and/or trace concentration. This drawback can be easily remediated through sensitivity-enhancement techniques. In particular, this paper discusses the fundamentals of surface plasmon resonance sensors and reviews the latest sensitivity enhancement methods such as the application of gold nanoparticles/nanostructures, magnetic nanoparticles and multilayer thin film structures. Specific recognition elements, when derivatized with nanoparticles, significantly increase the sensitivity of the SPR sensor. The main reason for sensitivity enhancement is the increase of analyte refractive index in the analyte–nanoparticle (NP) conjugates. Another important approach is the use of high refractive index thin layer films that lead to the enhancement of the surface plasmon electromagnetic field. These dielectric materials enhance the sensitivity of the SPR sensor through high absorption of the incident light and transfer it to the surface plasmons with minimum loss of energy. This review article provides a broad perspective on the state of the art of hypersensitive optical sensors.

Rapid extraction of trace bisphenol A in real water samples using hollow mesoporous silica surface dummy molecularly imprinted polymers by Leyan Li; Kai Yu; Mengjie Tian; Yafei Wang; Zulei Zhang; Guangyuan Jiang; Lei Li (3926-3932).
Hollow mesoporous silica surface dummy molecularly imprinted polymers (HM-DMIPs) were prepared via surface dummy molecular imprinting combined with a hybrid strategy for the highly selective recognition and rapid separation of trace bisphenol A (BPA) from environmental water samples. The obtained materials were well characterized using TEM, SEM, FT-IR, TG and N2 sorption analysis. Moreover, the proper binding and selective recognition ability were also investigated by a batch rebinding assay. As a control, hollow mesoporous silica surface molecularly imprinted polymers (HM-MIPs), silica surface dummy molecularly imprinted polymers (SiO2-DMIPs) and hollow mesoporous silica surface dummy non-imprinted polymers (HM-DNIPs) were also synthesized. Results showed that HM-DMIPs possessed a fast kinetics, high rebinding capacity of 69.6 mg g−1, 3.0, 2.2 and 4.7 times higher than that of SiO2-DMIPs, HM-MIPs and HM-DNIPs, respectively. Moreover, an extremely high selectivity toward BPA over 4,4′-biphenol, diethylstilbestrol and hydroquinone was obtained and the selectivity coefficients were all above 6.96. The HM-DMIPs could be reused for ten runs, indicating their excellent stability and reusability. The results of theoretical analysis indicated a chemical adsorption process. Furthermore, real water samples were successfully analyzed with HM-DMIPs and high recoveries in the range of 98.7–101.7% were obtained. These results demonstrated that HM-DMIPs could serve as an efficient sorbent for rapid separation of trace BPA from environmental water samples.

Upconversion nanoparticles (UCNPs) have attracted increasing interest owing to their excellent properties in chem/bio sensing. And DNA is the most frequently used biomolecule to functionalize the nanoparticles. However, there is still an urgent need for an effective and convenient strategy to assemble DNA strands on UCNPs. Herein, we utilized polydopamine (PDA) to coat hydrophobic UCNPs by a water-in-oil microemulsion method, which were then assembled with DNA through hydrogen bonding and π–π stacking interactions between the nucleotides of DNA and the aromatic groups of the PDA shell on the surface of the UCNPs. This strategy exhibited several merits such as decreasing the random coordination between phosphates of the DNA backbone and lanthanide ions of the UCNP surface and retaining the intrinsic DNA biorecognition functions for targets. A proof-of-concept biosensor for carcinoembryonic antigen (CEA) based on luminescence resonance energy transfer (LRET) was constructed using CEA aptamer-stabilized silver nanoparticles (Apt-AgNPs) as the energy acceptor. Due to the strong affinity between CEA and its aptamer, the upconversion luminescence was recovered in a [CEA]-dependent manner. This facile assembly strategy can be extended to various targets and sensors by altering the aptamer.

We report a simple fluorescence sensor array based on metal ions–gold nanoclusters (AuNCs) for the identification of proteins and bacteria. In this study, three protein-stabilized AuNCs including BSA-AuNCs, HSA-AuNCs and Lys-AuNCs were firstly synthesized. After the purification of these protein@AuNCs by Zn2+ and Cd2+ respectively, six types of metal ion-protein@AuNCs were employed as the sensing elements to fabricate a fluorescence sensor array. In the presence of a protein, differential changes of fluorescence intensity on sensing elements generate a unique response pattern for the protein. In combination with linear discriminant analysis (LDA), 9 proteins with different concentrations were differentiated successfully. Moreover, 5 different bacteria were also identified by the proposed sensor array. The method is simple, rapid, and sensitive and shows potential for application in biomolecule sensing.

Paper-based visual detection of silver ions and l-cysteine with a dual-emissive nanosystem of carbon quantum dots and gold nanoclusters by Bingyan Han; Ying Li; Xixi Hu; Qin Yan; Jingmei Jiang; Mingbo Yu; Tingting Peng; Gaohong He (3945-3950).
Fluorescence has been extensively employed for visual detection; however, it is inconvenient for people to distinguish single-color fluorescence changes. Herein, a dual-signal ratiometric fluorescent probe of a carbon quantum dot/gold nanocluster (CQD/Au NC) nanosystem for sensitive and selective detection of Ag+ and l-cysteine (Cys) was established. The CQDs with blue fluorescence at 472 nm served as a reference signal and Au NCs with orange fluorescence at 605 nm functioned as a reporter. From zeta potential measurements and TEM images, there was no indication of interaction between the two materials due to electrostatic repulsion. Upon adjusting the proportion of the CQDs and Au NCs, the nanosystem emitted a gray-orange emission at a single excitation wavelength. Ag+ enhanced the orange colour of Au NCs, but Cys could quench it completely with the high affinity of the coordination interaction between Cys and Ag+, while the blue fluorescence of CQDs was not affected. Therefore, the fluorescence of the nanosystem showed gray orange to light orange colour for determination of Ag+, and light orange to blue colour for determination of Cys. Moreover, paper-based visual detection was successfully carried out, which showed a more distinguishable color difference than that in aqueous solution. Our method has bright potential for application in the real-time and online visual detection of Ag+ and Cys in the future.

Dual function hollow structured mesoporous Prussian blue mesocrystals for glucose biosensors by Ke Zeng; Minghui Yang; You-Nian Liu; Avraham Rasooly (3951-3957).
Hollow structured mesoporous materials have attracted great interest in recent years. We present a new glucose biosensor based on dual function hollow structured mesoporous Prussian Blue (PB) mesocrystals (HMPB). HMPB serve as both a scaffold carrier matrix to immobilize the enzyme glucose oxidase (GOx-HMPB) on the electrode and as a redox mediator to H2O2, the by-product of GOx catalyzed glucose reaction. The Barrett–Joyner–Halenda (BJH) calculation based on nitrogen adsorption isotherm measurement indicates a ∼20 nm mesoporous outer shell and a ∼60 nm hollow cavity of HMPB that provide a large area (258 m2 g−1) for GOx loading. The larger surface area of HMPB compared to solid PB makes it much easier to reduce the HMPB at lower applied potential (0.130 V). The HMPB display enhanced sensitivity towards glucose detection due to increased GOx loading and increased catalytic sites of HMPB with the linear detection range from 0.05 to 7.3 mM, a limit of detection of 0.04 mM (S/N = 3) and a fast response time (within 6 s). The detected glucose level in human serum samples is in good agreement with the data from hospital clinical measurements. The data demonstrate that the HMPB mesocrystals acted effectively as both a redox mediator to H2O2 and a good enzyme carrier, suggesting that the strategy can be applied to other electroactive hollow mesoporous materials to prepare simplified biosensors for a wide range of clinical applications.

Nowadays, food contamination with pesticide residues is prevalent, which can cause problems to human health. For the analysis of these compounds in foods, multi-residue methods for specific matrices, which are not flexible, are normally developed. In this line, this study aimed to develop an analytical method using separation for ultra-fast liquid chromatography (UFLC) and analysis by high resolution mass spectrometry with ionisation by electrospray (ESI) and separation by quadrupole-time-of flight (ESI-Q-TOF) for the screening and confirmation of 93 pesticides in different kinds of foods (apple, lettuce and wheat flour). The method was based on QuEChERS (Quick Easy Cheap Effective Rugged Safe) with a subsequent dilution. This was validated in terms of linearity, precision and trueness, and limits of detection and quantification ranged from 1 to 10 μg kg−1 and 5 to 20 μg kg−1, respectively, with recoveries between 70 and 120% and a relative standard deviation (RSD) <20%. The compound confirmation was based on the exact mass as well as the isotope ratio. From sixteen real samples, three pesticide residues were detected in wheat flour samples, three in lettuce samples and four in apple samples. However, the concentrations detected (ranging from 2.7 to 408.0 μg kg−1) did not exceed the maximum residue limit (MRL) set by the European Union and Brazilian Health Surveillance Agency (ANVISA) through the Program for Analysis of Pesticide Residues (PARA). This practical method developed here was applied with success to different matrices for the analysis of pesticide residues in food with accuracy and sensitivity. Thus, a single method (which did not require complementary analysis) and an extraction procedure are useful for reducing pesticide identification errors in different food matrices.

Quantification of estrogens in infant formulas by isotope dilution liquid chromatography-tandem mass spectrometry by Shuan Liu; Xiaomin Li; Yingchen Zhao; Xiong Yin; Qinghe Zhang; Xiuqin Li; Guangshi Tang; Hongmei Li (3968-3975).
Steroid estrogens, such as estrone (E1), 17β-estradiol (E2) and estriol (E3), have been shown to exhibit potentially adverse effects on the health of children. Consequently, it is highly desirable to develop detection approaches for rapid and accurate determination of steroid estrogens in infant formulas. In this study, a robust method based on isotope dilution liquid chromatography-tandem mass spectrometry (ID LC-MS/MS) was developed to simultaneously measure E1, E2 and E3 in infant formulas with no need for any derivatization step. The desired simplicity, accuracy and specificity were achieved via a facile sample pretreatment process, including protein precipitation and clean-up steps. The limits of quantification were 0.0025, 0.005 and 0.005 μg kg−1 for E1, E2 and E3, respectively. Besides, a recovery of 92.3–102.7% was obtained with a relative standard deviation of less than 6%. Furthermore, both accurate values for the target analytes ranging from 0.005 to 50 μg kg−1 and a corresponding expanded relative uncertainty of less than 12% were obtained using the ID LC-MS/MS method. More importantly, the approach is highly sensitive, selective and accurate in screening estrogens in real infant formula samples. Thus, it could be convenient for large-scale application due to its derivatization-free feature and simplicity in sample preparation.

Degradation of RhB by a sono-Fenton-like process with an iron-foam in the presence of oxalic acid by Guangming Li; Shan Qiu; Fang Ma; Yawan Ji; Xiaofeng Jiang (3976-3983).
The decolorization of rhodamine B (RhB) using ultrasound (US) combined with a Fenton-like process has been investigated. The effects of operating parameters such as initial dye concentration, pH, and addition of oxalic acid were studied. As expected, the rate and extent of decolorization of RhB increased with rising oxalic acid concentration, but slightly decreased with increasing dye concentration, and exhibited excellent removal efficiency in the wide pH range of 3–9. It was found that, while the degradation rate due to ultrasound alone was slow, the sonication significantly accelerated the Fenton-like reaction. It has been observed that the maximum degradation of 92.16% was obtained under optimum operating conditions of [RhB] = 5 mg L−1; [Oxalic] = 25 mM; pH = 3. The decolorization pattern obeyed first order kinetics and was used to calculate apparent rate constants for dye decolorization. Furthermore, studies in the presence of radical scavengers such as tert-butyl alcohol (TBA), isopropanol (IPA), catalase and l-His on the degradation of RhB were performed, and the results indicated that and H2O2 played an important role in this system. Overall it can be said that the combined process resulted in higher extent of degradation as compared to the individual processes based on Fenton type or US irradiation.

A novel way for the determination of the acid values of edible oils using a phthalocyanine derivative by Ping Li; Fangdi Cong; Shulin Zhang; Haixue Liu; Yanling Xu; Jing Cui (3984-3990).
To effectively determine acid values (AVs) of edible oils by UV-vis spectroscopy, a novel method is developed based on the two characteristic peaks of tetra-α-octyloxy zinc phthalocyanine at 709 and 749 nm. They are sensitive to trace fatty acid in chloroform, and especially the one at 749 nm shows a good correlation between absorbance (AR) and fatty acid molar concentration (cb). The functional relationship can be expressed as AR = 0.08052 ln cb + 1.01100 suitable for common edible oils with AVs 0.06–2.40 mg KOH g−1. The analysis only needs 50 μL, about a drop, of edible oil mixed in 3 mL solution of the phthalocyanine compound in chloroform (1.0 × 10−5 mol L−1) to record AR and then calculate cb from which AV is determined. For the edible oils with larger AVs exceeding 2.40 mg KOH g−1, they can be diluted and analyzed by the UV-vis method. For those with minimal AVs no more than 0.06 mg KOH g−1, the AVs can be obtained by another supplementary equation AR = 3429.3cb + 0.073951, applicable to the range of 0.0024–0.06 mg KOH g−1. The UV-vis method is not only more convenient for the determination of AVs of edible oils but also hardly disturbed by their color in general, and thus it has great potential for application in the analysis of oil products.

A novel turn-on fluorescent probe (DDND) for highly selective detection of biothiols over other amino acids was synthesized and investigated in this work, which used the 2,4-dinitrobenzenesulfonyl (DNBS) group as a fluorescent quencher. The novel fluorophore (HDM) features a large π-conjugation system and a typical intramolecular charge transfer (ICT) process and has a long emission wavelength at 623 nm as well as a large Stokes shift (λem−λex = 131 nm). Besides that, this red-emitting probe exhibited good linearity ranges with a low detection limit of 0.23 μM for Cys, 0.34 μM for Hcy and 0.41 μM for GSH respectively. Upon titration of thiols, the color of the solution changed from yellow to dark red, which means it can be detected by naked eyes. Finally, probe DDND was successfully applied to bioimage intracellular Cys in HeLa cells with low cytotoxicity.

Automated 4-sample protein immunoassays using 3D-printed microfluidics by Karteek Kadimisetty; Andrew P. Spak; Ketki S. Bhalerao; Mohamed Sharafeldin; Islam M. Mosa; Norman H. Lee; James F. Rusling (4000-4006).
Low cost, miniaturized assay platforms that work with small sample volumes, high sensitivity and rapid detection will have high value in future biomolecular diagnostics. Herein we report an automated, 3D printed electrochemiluminescent (ECL) immunoarray integrated with a nanostructured pyrolytic graphite sheet (PGS) microwell chip configured to detect 2 proteins simultaneously from complex liquid samples with high sensitivity and selectivity. Assays are done in 18 min at cost of <$1.00 using 1–2 microliters of sample. 3D printed microfluidic array design integrates reagent and sample chambers with rapid ECL detection. A commercial programmable syringe pump used with a preset program allows pump to pause and resume reagent delivery as required for completion of the sandwich immunoassays. Nanostructured surfaces feature antibody-decorated single wall carbon nanotube forests on PGS chip microwells, and sensitivity is amplified via massively labeled RuBPY-silica nanoparticles for detection. Prostate specific antigen (PSA) and prostate specific membrane antigen (PSMA) were measured simultaneously from human serum on the immunoarray with detection limits 150 fg mL−1 for PSA and 230 fg mL−1 for PSMA, with dynamic ranges up to 5 ng mL−1. Validation of the immunoarray by measuring these proteins in human serum showed good correlation with single protein ELISA. These 3D printed platforms can be easily adapted to multiple applications and configurable CAD files for the immunoarray can be downloaded from our lab's website.

Back cover (4007-4008).