Showing papers in "Mikrochimica Acta in 2017"

The quenching of the fluorescence of carbon dots: A review on mechanisms and applications

Abstract: Carbon dots (CDs) possess unique optical properties such as tunable photoluminescence (PL) and excitation dependent multicolor emission. The quenching and recovery of the fluorescence of CDs can be utilized for detecting analytes. The PL mechanisms of CDs have been discussed in previous articles, but the quenching mechanisms of CDs have not been summarized so far. Quenching mechanisms include static quenching, dynamic quenching, Forster resonance energy transfer (FRET), photoinduced electron transfer (PET), surface energy transfer (SET), Dexter energy transfer (DET) and inner filter effect (IFE). Following an introduction, the review (with 88 refs.) first summarizes the various kinds of quenching mechanisms of CDs (including static quenching, dynamic quenching, FRET, PET and IFE), the principles of these quenching mechanisms, and the methods of distinguishing these quenching mechanisms. This is followed by an overview on applications of the various quenching mechanisms in detection and imaging.

Nanocomposites of graphene and graphene oxides: Synthesis, molecular functionalization and application in electrochemical sensors and biosensors. A review

Abstract: Functionalized nanocomposites based on various type of graphene nanomaterials including graphene, graphene oxides (GOs), and doped graphene (oxides) are widely used as materials for various sensors that can display high sensitivity, selectivity and stability. This review with 347 references summarizes advances in the preparation and functionalization of graphene nanocomposites for the application of electrochemical sensors and biosensors. Following a general introduction into the field, the article is divided into subsections on (a) the synthesis and functionalization of nanocomposites (made from graphene, various kinds of GOs, heteroatom-doped GOs), (b) on methods for functionalization of composites (with other carbon nanomaterials, metal nanoparticles, metal oxide and metal sulfide nanoparticles), (c) on functionalization with inorganic materials including polyoxometalates, hexacyanoferrates, minerals), (d) on functionalization with organic materials such as amino acids, surfactants, organic dyes, ionic liquids, macrocycles (including cyclodextrins, crown ethers and calixarenes), and (e) on functionalization with organometallics and with various other organic compounds, (f) on functionalizations with polymers such as conventional polymers, polyelectrolytes, conducting polymers, molecularly imprinted polymers, (g) on functionalization with biomolecules including proteins and nucleic acids. Other subsections cover flexible graphene and GO based nanocomposites and 3D composites. Application of graphene and GO nanocomposites are then covered in a in large section that comprises electrochemical sensors and biosensors (based on voltammetry, amperometry, potentiometry, impedimetry, electrochemiluminescence, photoelectrochemistry, field effect transistors, electrochemical immunosensors) with specific subsections on gas sensors, enzymatic biosensors and gene sensors. A concluding section covers current challenges and perspectives of graphene and GO based (bio)sensing.

Carbon dots doped with heteroatoms for fluorescent bioimaging: a review

Abstract: Carbon dots (CDs) possess superior fluorescent properties in that they do not blink, are biocompatible, chemically inert, have small size and well tunable photoluminescence (PL), can be easily functionalized with biomolecules, and can be multi-photon excited to give up-converted PL. This review (with 141 refs.) summarizes recent progress in the field of imaging using carbon dots doped with heteroatoms (X-CDs). Following an introduction, we discuss top-down and bottom-up strategies for synthesis and methods for surface modification. We also compare the differences in synthesis for undoped CDs and X-CDs. Specifically, CDs doped with heteroelemets nitrogen, phosphorus, sulfur, selenium, boron and silicium are treated. We then discuss method for determination of the properties (particle size, ZP), how doping affects fluorescence (spectra, quantum yields, decay times), and how dopants affect upconversion (UC, anti-Stokes luminescence). We finally review the progress made in fluorescent imaging of cells tissue, and other biomatter. This review also gives new hints on how to use synthetic methods for tuning the structure of X-CDs, how doping affects properties, and how to achieve new bioimaging applications.

An overview on enzyme-mimicking nanomaterials for use in electrochemical and optical assays

Abstract: Various kinds of nanomaterials have been described in recent years that represent stable and low-cost alternatives to biomolecules (such as enzymes) for use in (bio)analytical methods. The materials typically include, metal/metal oxides, metal complexes, nanocomposites, porphyrins, phthalocyanines, smart polymers, and carbonaceous nanomaterials. Due to their biomimetic and other properties, such nano-materials may replace natural enzymes in chemical sensors, biosensors, and in various kinds of bioassays. This overview (with 252 references) highlights the analytical potential of such nanomaterials. It is divided into sections on (a) the types of nanomaterials according to their intrinsic nature, (b) non-enzymatic sensor designs (including electrochemical, colorimetric, fluorescent and chemiluminescent methods), and (c), applications of non-enzymatic sensors in the biomedical, environmental and food analysis fields. We finally address current challenges and future directions.

A bimetallic (Co/2Fe) metal-organic framework with oxidase and peroxidase mimicking activity for colorimetric detection of hydrogen peroxide

Abstract: A bimetallic metal-organic framework of the type MOF(Co/2Fe), synthesized by a hydrothermal method, is demonstrated to exhibit peroxidase-like activity and oxidase-like activity. The material catalyzes the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) to produce a blue product both in the absence and presence of H2O2. Its catalytic activity strongly depends on temperature and the solution pH value. With its dual enzyme mimetic activity, MOF(Co/2Fe) has the advantages of low cost, good stability, ease of preparation, and attractive Michaelis-Menten behavior. The mechanisms and kinetics of the pseudo-enzymatic activity were studied in some detail. A colorimetric method was developed for determination of H2O2 that is based on the peroxidase-like activity of MOF(Co/2Fe). Under the optimal conditions, the absorbance of oxidized TMB at 652 nm increases linearly in the 10 to 100 μM H2O2 concentration range, with a 5 μM limit of detection.

A review on nanomaterial-based electrochemical sensors for H2O2, H2S and NO inside cells or released by cells

Abstract: The incorporation of nanomaterials into electrochemical sensors is an attractive approach towards the improvement of the sensitivity of amperometry and also can provide improved sensor selectivity and stability. This review (with 137 references) details the current state of the art and new trends in nanomaterial-based electrochemical sensing of hydrogen peroxide (H2O2), hydrogen sulfide (H2S) and nitric oxide (NO) in cells or released by cells. The article starts with a discussion of the significance of the three analytes, and this is followed by three sections that summarize the electrochemical detection schemes for H2O2, H2S and NO. Each section first summarizes the respective physiological roles, and then reviews electrochemical sensors based on the use of carbon nanomaterials, noble metal nanomaterials, metal oxide nanomaterials, and layered doubled hydroxides. The materials are compiled in three tables along with figures of merit for the various sensors.

A water-stable lanthanide metal-organic framework for fluorimetric detection of ferric ions and tryptophan

Abstract: The preparation of a highly water stable and porous lanthanide metal-organic framework (MOF) nanoparticles (denoted SUMOF-7II; SU refers to Stockholm University) is described. SUMOF-7II was synthes ...

Water compatible molecularly imprinted nanoparticles as a restricted access material for extraction of hippuric acid, a biological indicator of toluene exposure, from human urine

Abstract: The authors describe the preparation of molecularly imprinted nanoparticles (MINPs) for the solid phase extraction of hippuric acid (HA). The MINPs consists of a water-compatible organic-inorganic silica composite which was obtained from a functionalized silica by the sol-gel method. HA acted as the template, 3-aminopropyl trimethoxysilane as the functional monomer, and tetraethoxysilane as the crosslinker. Subsequently, methacryloxypropyltrimethoxysilane was used as a coupling agent to deposit a hydrophilic acrylamide coating onto the surface of the MINPs. The morphology and structure of the resulting restricted access material (referred to as RAM-MINP) were characterized by scanning electron microscopy, transmission electron microscopy and Fourier transform infrared spectroscopy. Solid phase extraction of HA was accomplished by passing urine samples through a RAM-MINP-packed SPE cartridge. Following elution, HA was quantified by HPLC using UV detection at 228 nm. The effects of sample pH, amount of sorbent and eluent and washing solvent volumes were optimized by experimental design methodology under response surface methodology. Under optimized conditions, the mean extraction efficiency of HA from spiked samples is adequately repeatable, with relative standard deviations of <6.1%. The limits of detection and quantitation are 0.15 and 0.25 μg.L−1, respectively.

Nitrogen and sulfur co-doped carbon quantum dots for highly selective and sensitive fluorescent detection of Fe(III) ions and L-cysteine

Abstract: Nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) with a high quantum yield (69%) and excellent photoluminescent properties were synthesized via a facile hydrothermal method using citric acid and cysteine as precursors. The cytotoxicity of N,S-CQDs was evaluated through the MTT assays using HepG2 cells as the target. The cell viability is still >95% after a 24 h incubation with particles in a concentration of up to 300 μg·mL−1, thereby indicating low cytotoxicity and excellent biocompatibility of N,S-CQDs. The emission spectra of the N,S-CQDs are nearly independent of the excitation wavelengths in the range from 280 to 380 nm and have the strongest fluorescence emission centered at 415 nm. Co-doping with N and S promotes the electron-transfer rate and coordination interaction between N,S-CQDs and Fe(III) ions which acts as a quencher of fluorescence. Fluorescence can be recovered, however, by addition of L-Cys. Hence, the N,S-CQDs act as a highly sensitive and selective “turn-off-on” probe for the determination of Fe(III) or Cys. The limits of detection are 14 nM for Fe(III) and 0.54 μM for Cys. The interferences by 17 common metal ions and heavy metal ions, 12 other quenchers and 16 amino acids were investigated and found to be tolerable. The probe was successfully applied for cellular imaging of Fe(III) in HepG2 cells by using fluorescence microscopy.

Electrochemical DNA sensors based on the use of gold nanoparticles: a review on recent developments

Abstract: Electrochemical DNA sensors represent a simple, accurate and economical platform for DNA detection. Gold nanoparticles are known to be efficient labels in electrochemical sensors and to be viable materials to modify the surface of electrodes thereby to enhance the detection limit of the sensor. For surface modification, gold nanoparticles are used in combination with nanomaterials like graphene, graphene oxide, or carbon nanotubes to improve electrochemical performance in general. This review (with 116 refs.) mainly covers the advances made in recent years in the use of gold nanoparticles in DNA sensing. It is divided into the following main sections: (a) An introduction covers aspects of electrochemical sensing of DNA and of appropriate nanomaterials in general. (b) The use of gold nanoparticles in DNA is specifically addressed next, with subsections on AuNPs acting as electrochemical labels, electron transfer mediators, signal amplifiers, carriers of electroactive molecules, catalysts, immobilization platforms, on silver enhancement strategies, on AuNPs modified with carbonaceous materials (such as graphenes and nanotubes), and on multiple amplification schemes. The review concludes with a discussion of current challenges and trends in terms of highly sensitive DNA based sensing using AuNPs.

Achievement and assessment of direct electron transfer of glucose oxidase in electrochemical biosensing using carbon nanotubes, graphene, and their nanocomposites

Abstract: Carbon nanotubes, graphenes, and their hybridized composites with nanoparticles have been attempted to establish a direct electrical communication between the recognition biomolecule and its underlying electrode surface. This review (with 133 refs.) focuses on advances, strategies and technical challenges in the development of reagentless electrochemical biosensors for glucose with enhanced detection sensitivity, selectivity, and simplicity. Specifically, the review commences with a discussion of the relevance of direct electron transfer (DET) in biosensing together with the fundamental of electro-enzymology and kinetics. General aspects of glucose oxidase (GOx), the most popular enzyme with a flavin cofactor, are discussed in view of its historical and important role in the development of electrical biosensors for blood glucose. The next section assesses DET of GOx based on the Marcus theory and the Laviron formalism. The reorganizational energy of the Marcus model and the overpotential play an important role in reaction kinetics and affect the rate of electron transfer significantly. The presence of nanomaterials, particularly for graphene oxide, decreases the electron transfer distance between the enzyme redox center and the underlying electrode surface well beyond 15 A. The improper Marcus-Hush-Chidsey integral is now simplified to estimate the rate of electron transfer with very good accuracy. Critiques, technical challenges, and future possibilities of glucose electrodes with respect to DET are also presented and discussed.

Nanocomposites composed of layered molybdenum disulfide and graphene for highly sensitive amperometric determination of methyl parathion

Abstract: The authors describe an inexpensive electrode for the sensitive amperometric determination of the pesticide methyl parathion. A glassy carbon electrode was modified with a nanocomposite consisting of molybdenum disulfide nanosheets (MoS2) and graphene that was prepared via a hydrothermal process. Its morphology, elemental composition, diffraction, impedance and voltammetric characteristics were studied. The modified electrode displays excellent electrocatalytic ability towards methyl parathion, and the reduction peak current, measured typically at −0.60 V (vs. Ag/AgCl) is related to the concentration of methyl parathion. The effect of concentration, scan rate and solution pH value were optimized. The calibration plot is linear in the 10 nM to 1.9 mM concentration range, with a 3.2 nM detection limit (at a signal-to-noise ratio of 3). The electrode is selective, stable, adequately repeatable and reproducible. The method was successfully applied to the determination of methyl parathion in spiked samples of homogenized apple, kiwi, tomato and cabbage.

Fluorometric determination of the antibiotic kanamycin by aptamer-induced FRET quenching and recovery between MoS 2 nanosheets and carbon dots

Abstract: A rapid and sensitive aptamer-based assay is described for kanamycin, a veterinary antibiotic with neurotoxic side effects. It is based on a novel FRET pair consisting of fluorescent carbon dots and layered MoS2. This donor-acceptor pair (operated at excitation/emission wavelengths of 380/440 nm) shows fluorescence recovery efficiencies reaching 93 %. By taking advantages of aptamer-induced fluorescence quenching and recovery, kanamycin can be quantified in the of 4–25 μM concentration range, with a detection limit of 1.1 μM. The method displays good specificity and was applied to the determination of kanamycin in spiked milk where it gave recoveries ranging from 85 % to 102 %, demonstrating that the method serves as a promising tool for the rapid detection of kanamycin in milk and other animal-derived foodstuff.

Amino-functionalized MIL-101(Fe) metal-organic framework as a viable fluorescent probe for nitroaromatic compounds

Abstract: A highly luminescent iron(III)-based amino-functionalized metal-organic framework (MOF) of type NH2-MIL-101(Fe) was synthesized by a solvothermal method. Its structure and morphology were studied by X-ray diffraction, scanning electron microscopy, and FTIR. The strong fluorescence of the electron-rich MOF is shown to be quenched by electron-deficient nitroaromatic compounds. Compared to solvents such as water and other organic solvents, quenching by the nitroaromatic compounds nitrobenzene (NB), 4-nitrophenol (4-NP), 4-nitrotoluene (4-NT) and 1,3-dinitrobenzene (1,3-DNB) is particularly strong. The detection limits for NB, 4-NP, 4-NT and 1,3-DNB are 32 ppm, 17 ppm, 9.8 ppm and 11.5 ppm.

A covalent organic framework-based magnetic sorbent for solid phase extraction of polycyclic aromatic hydrocarbons, and its hyphenation to HPLC for quantitation

Abstract: A novel covalent organic framework based magnetic adsorbent was developed for magnetic solid phase extraction (MSPE) of polycyclic aromatic hydrocarbons (PAHs). Covalent organic framework-LZU1 (= Lan Zhou University-1) was covalently immobilized onto polyethyleneimine-functionalized magnetic nanoparticles (COF-LZU1@PEI@Fe3O4), and the resulting material was characterized by transmission electron microscopy and Fourier transform infrared spectroscopy. The effects of the pH value of sample solution, percentage of acetonitrile, extraction time and sampling volume on MSPE of six PAHs were investigated. The COF-LZU1@PEI@Fe3O4 displays high extraction efficiency for the PAHs such as pyrene, benzo[a]pyrene, fluoranthene, benz[a]anthracene, benzo[a]fluorathene and dibenz[a,h]anthracene. Following desorption with acetonitrile, the PAHs were quantified by HPLC. The MSPE-HPLC method shows low limit of detection (0.2–20 pg mL−1), wide linear range and good reproducibility (relative standard deviations <4.4% for intra-day and inter-day precision). The method was successfully applied to determine PAHs in environmental samples. Good recoveries were obtained, ranging from 90.9 to 107.8% for water samples and 85.1 to 105.0% for soil samples.

Optical sensing of temperature based on non-thermally coupled levels and upconverted white light emission of a Gd2(WO4)3 phosphor co-doped with in Ho(III), Tm(III), and Yb(III)

Abstract: The authors describe a new phosphor of the composition Gd2(WO4)3:0.3%Ho/0.5%Tm/15%Yb that displays distinctly improved sensitivity to temperature (T) due to non-thermally coupled energy levels of Ho(III) and Tm(III) ions. Under 980 nm excitation, the upconverted blue (peaking at 473 nm), green (540 nm) and red (655 and 700 nm) fluorescences exhibit T-dependent luminescence in the 295 to 595 K range. The T sensing capability was investigated by using a ratiometric (2-wavelength) technique. The maximum sensitivity to T is found to be 26.14 × 10−3 K−1 at 595 K based on the red [Tm(III)] and green [Ho(III)] emissions. The colors of the phosphor are located in the white region of the CIE diagram in the whole T-range which also makes it suitable for use in white displays.

Nanomaterial-based biosensors for detection of prostate specific antigen.

Abstract: Screening serum for the presence of prostate specific antigen (PSA) belongs to the most common approach for the detection of prostate cancer. This review (with 156 refs.) addresses recent developments in PSA detection based on the use of various kinds of nanomaterials. It starts with an introduction into the field, the significance of testing for PSA, and on current limitations. A first main section treats electrochemical biosensors for PSA, with subsections on methods based on the use of gold electrodes, graphene or graphene-oxide, carbon nanotubes, hybrid nanoparticles, and other types of nanoparticles. It also covers electrochemical methods based on the enzyme-like activity of PSA, on DNA-, aptamer- and biofuel cell-based methods, and on the detection of PSA via its glycan part. The next main section covers optical biosensors, with subsections on methods making use of surface plasmon resonance (SPR), localized SPR and plasmonic ELISA-like schemes. This is followed by subsections on methods based on the use of fiber optics, fluorescence, chemiluminescence, Raman scattering and SERS, electrochemiluminescence and cantilever-based methods. The most sensitive biosensors are the electrochemical ones, with lowest limits of detection (down to attomolar concentrations), followed by mass cantilever sensing and electrochemilumenescent strategies. Optical biosensors show lower performance, but are still more sensitive compared to standard ELISA. The most commonly applied nanomaterials are metal and carbon-based ones and their hybrid composites used for different amplification strategies. The most attractive sensing schemes are summarized in a Table. The review ends with a section on conclusions and perspectives.

Recent advances in optical detection of dopamine using nanomaterials

Abstract: This review (with 121 references) gives a summary on recent advances in the use of nanomaterials for the optical determination of dopamine. It includes a brief overview of the clinical significance of dopamine followed by a discussion on the recent advances in various nanomaterial-based distinctive optical spectroscopic methods such as (a) colorimetry and spectrophotometry, (b) surface-enhanced Raman spectroscopy (SERS), (c) fluorescence spectrometry, and (d) electrochemiluminescence (ECL) spectrometry. All sections are further divided into subsections based on the type of nanomaterial used, and their advantages and disadvantages are discussed. A discussion on the validity of the nanomaterial-based optical detection of dopamine for human samples is also included. This review concludes with highlights of current challenges of nanomaterial-based optical sensors and an outlook on future perspectives.

Paper-based device with on-chip reagent storage for rapid extraction of DNA from biological samples

Abstract: Conventional methods for extraction of DNA are expensive, time-consuming and tedious. To overcome these limitations, a paper-based DNA extraction device was developed that incorporates sponge-based buffer storage, a paper-based valve and channels of different length to autonomously direct the reagent and sample to the Fusion 5 disk for DNA capturing. With this device, DNA can be extracted within 2 min from only 30 μL samples of whole blood, serum, breast cancer cell, saliva, sputum and bacterial suspension. The device can also extract Hepatitis B Virus DNA from clinical blood samples and after quantification shows a detection limit as low as 104 copies⋅mL−1. This highlights its potential use in future diagnostics. The performance of the device was similar to that of the commercial QIAamp DNA micro kits and the FTA card. In our perception, this simple, inexpensive, portable and disposable device holds great promise in terms of POC testing in resource-poor settings.

Gold and silver nanoparticles and indicator dyes as active agents in colorimetric spot and strip tests for mercury(II) ions: a review

Abstract: This review with 111 references covers recent progress made in the field of strip tests and spot tests for quantitative determination of mercury(II) ions. Following an introduction into the subject and the fundamentals of colorimetric determination, we first cover methods for synthesis and characterization of gold and silver nanoparticles (NPs) and give representative examples. The next sections cover (a) methods for dye-based detection of Hg(II) ions (categorized into azo, acridine, anthraquinone, di−/triarylmethane, nitro/nitroso, cyanine, triazole, oxazine, thiazine, dioxazine and xanthene dyes), and (b) techniques for immobilization of active agents (NPs and indicators) on the solid support. A conclusion section discusses current challenges and trends in future research.

Chitosan-wrapped gold nanoparticles for hydrogen-bonding recognition and colorimetric determination of the antibiotic kanamycin

Abstract: The authors describe a colorimetric assay for the quantitation of antibiotic kanamycin. It is based on hydrogen-bonding recognition capability of gold nanoparticles (AuNPs) in a chitosan matrix. Hydrogen-bonding interaction between kanamycin and chitosan induces the aggregation of the AuNPs, and this result in a color change of the AuNPs from wine red to blue. The ratio of the absorbance at 650 nm and 520 nm increases linearly over the 0.01 to 40 μM kanamycin concentration range, and the detection limit is 8 nM (at an S/N ratio of 3). This low detection limit easily matches the maximum residue limits established by the European Union for milk (288 nM). Selectivity experiments revealed the assay to have a satisfactory specificity. It was applied to the determination of trace levels of kanamycin in (spiked) real samples. Results agreed well with those obtained by HPLC. The method does not require tedious detection procedure or sophisticated instrumentation.

Magnetic metal-organic frameworks for the extraction of trace amounts of heavy metal ions prior to their determination by ICP-AES

Abstract: The authors describe the preparation of two kinds of metal-organic frameworks (MOFs), referred to as TMU-8 and TMU-9. The MOFs were applied to the preconcentration of the ions Co(II), Cu(II), Pb(II), Cd(II), Ni(II), Cr(III), and Mn(II) from aqueous solutions. The roles of the azine groups in TMU-8 (in comparison to TMU-9 which does not have an azine group) and the role of void spaces of these MOFs toward the adsorption of metal ions also are evaluated. The studies reveal that TMU-8 has a better adsorption capability than TMU-9. A magnetic TMU-8 was then fabricated by in-situ synthesis of a magnetic core-shell nanocomposite. The material was chosen as an efficient sorbent for the preconcentration of the above metal ions, followed by their determination by flow injection inductively coupled plasma atomic emission spectrometry. The assay was optimized using a combination of central composite design (CCD) and a Bayesian regularized artificial neural network (BRANN) technique. Under optimal conditions, the preconcentration factors are in the range between 66 and 232, and detection limits are as low as 0.3 to 1 μg ⋅L−1. The relative standard deviations are <6.4% (for n = 3; at 50 μg ⋅ L−1). Real samples were analyzed, and the results demonstrate that such core-shell magnetic microspheres are promising sorbents for rapid and efficient extraction of heavy metal ions from complex samples.

Hierarchical C-N doped NiO with dual-head echinop flowers for ultrasensitive monitoring of epinephrine in human blood serum

Abstract: The authors describe nanoelectrodes based on the use of hierarchical carbon-nitrogen nanospheres and dual-head nickel oxide echinop flowers (CN@HDN) placed on indium tin oxide (ITO) electrodes. The modified electrodes enable sensitive detection of catecholamine neurotransmitters, specifically of epinephrine (EPI) in human serum samples. The modified electrodes possess many active sites along the {111} crystal plane and large contact surfaces. This enables a rapid EPI diffusion within a highly active transport surface. The geometrical and morphological structures of the NiO decorated with CN-nanospheres render superior electrocatalytic behavior at a relatively low working voltage of 0.12 V (vs. Ag/AgCl) which makes the sensor relatively specific. The use of CN also increases the electron transfer rate and facilitates mass transfer between electrolyte (EPI sample) and catalytically active sites. The electrode is sensitive, selective and works at near-physiological pH values. It has a detection limit as low as 4 nM of EPI.

Dually emissive P,N-co-doped carbon dots for fluorescent and photoacoustic tissue imaging in living mice

Abstract: Hydrothermal treatment of a mixture of ethylene diamine, phosphoric acid and citric acid under ambient pressure generates fluorescent carbon dots that are co-doped with phosphorus and nitrogen. These have features such as (a) both green fluorescence (peaking at 430 nm; 30% quantum yield) and red fluorescence (peaking at 500 nm, quantum yield 78%), (b) wavelength-dependent emission peaks, and (c) insensitivity to changes of pH values, dot concentration and ionic strength. The C-dots are useful for both fluorescent (FL) and photoacoustic (PA) imaging of living tissue. PA imaging warrants better spatial resolution and allows deeper tissues to be imaged compared to most optical imaging techniques. It is essential to assign a photoacoustic contrast agent as most of the diseases do not show a natural photoacoustic contrast in their early stage. The dually emitting C-dots are shown to be a useful contrast agent for PA and FL imaging of mice tumors. Intravenous administration of the C-dots resulted in strong signals in both PA and FL imaging.

A sensitive colorimetric assay for cholesterol based on the peroxidase-like activity of MoS2 nanosheets

Abstract: The authors describe a colorimetric method for the determination of cholesterol in human serum. Cholesterol is enzymatically oxidized by oxygen in the presence of cholesterol oxidase (ChOx) to produce 4-cholestene-3-one and hydrogen peroxide (H2O2). Due to the peroxidase-like activity of MoS2 nanosheets, H2O2 can oxidize the chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB) to give a blue product. The increase in the absorbance of the acidic oxTMB solution at 450 nm is used for quantification of cholesterol. Under the optimal conditions, the increase in absorbance is proportional to the concentration of cholesterol in the range from 2 to 200 μmol⋅L−1, and the limit of detection is 0.76 μmol⋅L−1. The color change from pale yellow to blue color can also be detected visually for cholesterol concentrations as low as 20 μmol⋅L−1. Such an enzyme mimetic-based assay possesses advantages over enzyme-based assays in terms of costs, stability against denaturation, and protease digestion. The assay was applied to the determination of free cholesterol and of total cholesterol (after hydrolysis using an esterase) in spiked human serum, and it gave satisfactory recoveries.

Amperometric aptasensor for ochratoxin A based on the use of a gold electrode modified with aptamer, complementary DNA, SWCNTs and the redox marker Methylene Blue

Abstract: The authors describe an amperometric aptasensor for the mycotoxin ochrotoxin A (OTA). It is based on the use of a modified gold electrode containing aptamer (Apt) as the sensing ligand, Methylene Blue (MB) as the redox indicator, single-walled carbon nanotubes (SWCNTs) as electrochemical signal amplifiers, and complementary strands of aptamer (CSs) as assisting DNA. In the absence of OTA, the duplex formed between Apt and CSs on the electrode remains intact. Thus, a strong electrochemical signal is observed due to the presence of the redox marker MB in the duplex. If OTA is added, the duplex will be disassembled and MB and SWCNTs will be released from the surface of the gold electrode. Hence, the electrochemical signal is weakened. The method is highly specificity for OTA and has a limit of detection as low as 52 pM. The aptasensor was successfully applied to the determination of OTA in (spiked) serum and grape juice samples where it shows LODs of 134 and 58 pM, respectively.

Advanced nanomaterials for use in electrochemical and optical immunoassays of carcinoembryonic antigen. A review

Abstract: This review (with 196 refs.) covers the state of the art in electrochemical and optical immunoassays for the carcinoembryonic antigen (CEA). In essence, it has sections on (a) frequently applied principles and types of CEA immunoassays; (b) aspects of sensor fabrication including immunological and immobilization procedures and the proper choice of nanomaterials; (c) electrochemical immunoassays, with subsections on assays based on the use of nanoparticles and other nanomaterials (such as conducting polymers and graphenes); (d) optical immunoassays based on the use of nanoparticles such as quantum dots, gold nanoparticles, upconversion nanoparticles, graphenes and their derivatives; (d) lateral flow and lab-on-a-chip (microfluidic) immunoassays; and (e) on multiplexed electrochemical and optical immunoassays with and without labels. Examples for applications to real samples are given. A final section discusses current limitations and trends in terms of sensing schemes and nanomaterials.

Highly N,P-doped carbon dots: Rational design, photoluminescence and cellular imaging

Abstract: The authors describe the synthesis of carbon dots (CDs) that are highly doped with both nitrogen and phosphorus. Synthesis is accomplished via a hydrothermal reaction starting from diethylenetriaminepenta(methylenephosphonic acid) and m-phenylenediamine as the precursors. The high N,P-doping ratios renders the codoped CDs excellently water soluble, photostable over a wide range of pH values, and photostable in the presence of various metal ions. Ferric ions acts as a strong quencher of fluorescence. Their low cytotoxicity and strong green fluorescence (with excitation/emission peaks at 440/510 nm and a quantum yield of 0.32) make the CDs well suited for purposes of cell imaging, and this is demonstrated by fluorescent bioimaging of human lung carcinoma cells (type A549) and human breast cancer cells (type KB). Furthermore, the CDs were used as an effective probe for monitoring Fe(III) in both aqueous solution and living cells.

Magnetic solid phase extraction of non-steroidal anti-inflammatory drugs from water samples using a metal organic framework of type Fe3O4/MIL-101(Cr), and their quantitation by UPLC-MS/MS

Abstract: The authors describe a method for magnetic solid phase extraction (MSPE) combined with ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) for the quantitation of eight non-steroidal anti-inflammatory drugs (NSAIDs) in wastewater and environmental water samples. Specifically, the NSAIDs were directly extracted by using a magnetic nanocomposite of type Fe3O4/MIL-101(Cr) (Material Institute Lavoisier, MIL) from 50 mL water samples without any further cleanup. The effects of the amount of adsorbent, pH value, salt concentration, ultrasonic extraction time, desorption solvent nature and volume and desorption time were investigated. Under the optimal conditions, UPLC-MS/MS based detection resulted in the following figures of merit: (a) Linear calibration plots that typically extend from 0.1 (0.2, 0.05, 0.02) to 50 μg·L−1, depending on the compound; (b) detection limits between 3 to 60 ng·L−1; and (c), recoveries varying from 81.2% to 116.8% with relative standard deviations between 0.4% and 10.8%. The adsorption of NSAIDs on Fe3O4/MIL-101(Cr) was characterized by adsorption isotherms, kinetics and thermodynamics studies and provides a deeper insight into the adsorption mechanism. Compared to other method for pretreatment and preconcentration of NSAIDs, the new method is more rapid, sensitive, accurate and eco-friendly. The method was successfully applied to the determination of the NSAID residues in a variety of real environmental and wastewater samples.

Multicolor immunochromatographic strip test based on gold nanoparticles for the determination of aflatoxin B1 and fumonisins

Abstract: The authors describe the preparation of desert rose-like gold nanoparticles (DR-GNPs) with a plasmon resonance band at 620 nm which gives them a blue color. They have a hydrodynamic diameter of ∼72 nm and were prepared by a seeding growth approach. The DR-GNPs were characterized by UV-vis spectroscopy, transmission electron microscopy and dynamic light scattering. These nonspherical GNPs were used as a label for the antibody in an immunochromatographic strip test (ICST). Despite their particular shape and the higher surface area compared to spherical gold nanoparticles, the DR-GNPs are useful blue labels for the GNP-based strip test. A multicolor ICST for aflatoxin B1 and fumonisins is described that employs both blue DR-GNPs and red spherical GNPs. It allows for simultaneous rapid determination of the two mycotoxins in maize flour, with visual cut-off levels of 2 μg⋅kg-1 for aflatoxin B1 and of 1000 μg⋅kg-1 for fumonisins.