Qingsong Mei

Instant Visual Detection of Trinitrotoluene Particulates on Various Surfaces by Ratiometric Fluorescence of Dual-Emission Quantum Dots Hybrid

To detect trace trinitrotoluene (TNT) explosives deposited on various surfaces instantly and on-site still remains a challenge for homeland security needs against terrorism. This work demonstrates a new concept and its utility for visual detection of TNT particulates on various package materials. The concept takes advantages of the superior fluorescent properties of quantum dots (QDs) for visual signal output via ratiometric fluorescence, the feasibility of surface grafting of QDs for chemical recognition of TNT, and the ease of operation of the fingerprint lifting technique. Two differently sized CdTe QDs emitting red and green fluorescences, respectively, have been hybridized by embedding the red-emitting one in silica nanoparticles and covalently linking the green-emitting one to the silica surface, respectively, to form a dual-emissive fluorescent hybrid nanoparticle. The fluorescence of red QDs in the silica nanoparticles stays constant, whereas the green QDs functionalized with polyamine can selectively bind TNT by the formation of Meisenheimer complex, leading to the green fluorescence quenching due to resonance energy transfer. The variations of the two fluorescence intensity ratios display continuous color changes from yellow-green to red upon exposure to different amounts of TNT. By immobilization of the probes on a piece of filter paper, a fingerprint lifting technique has been innovated to visualize trace TNT particulates on various surfaces by the appearance of a different color against a yellow-green background under a UV lamp. This method shows high selectivity and sensitivity with a detection limit as low as 5 ng/mm(2) on a manila envelope and the attribute of being seen with the naked eye.

Ligand Replacement-Induced Fluorescence Switch of Quantum Dots for Ultrasensitive Detection of Organophosphorothioate Pesticides

The development of a simple and on-site assay for the detection of organophosphorus pesticed residues is very important for food safety and exosystem protection. This paper reports the surface coordination-originated fluorescence resonance energy transfer (FRET) of CdTe quantum dots (QDs) and a simple ligand-replacement turn-on mechanism for the highly sensitive and selective detection of organophosphorothioate pesticides. It has been demonstrated that coordination of dithizone at the surface of CdTe QDs in basic media can strongly quench the green emission of CdTe QDs by a FRET mechanism. Upon the addition of organophosphorothioate pesticides, the dithizone ligands at the CdTe QD surface are replaced by the hydrolyzate of the organophosphorothioate, and hence the fluorescence is turned on. The fluorescence turn on is immediate, and the limit of detection for chlorpyrifos is as low as ∼0.1 nM. Two consecutive linear ranges allow a wide determination of chlorpyrifos concentrations from 0.1 nM to 10 μM. Importantly, the fluorescence turn-on chemosensor can directly detect chlorpyrifos residues in apples at a limit of 5.5 ppb, which is under the maximum residue limit allowed by the U.S. Environmental Protection Agency. The very simple strategy reported here should facilitate the development of fluorescence turn-on chemosensors for chemo/biodetection.

Inkjet-Printed Silver Nanoparticle Paper Detects Airborne Species from Crystalline Explosives and Their Ultratrace Residues in Open Environment

An electronic nose can detect highly volatile chemicals in foods, drugs, and environments, but it is still very much a challenge to detect the odors from crystalline compounds (e.g., solid explosives) with a low vapor pressure using the present chemosensing techniques in such way as a dogs olfactory system can do. Here, we inkjet printed silver nanoparticles (AgNPs) on cellulose paper and established a Raman spectroscopic approach to detect the odors of explosive trinitrotoluene (TNT) crystals and residues in the open environment. The layer-by-layer printed AgNP paper was modified with p-aminobenzenethiol (PABT) for efficiently collecting airborne TNT via a charge-transfer reaction and for greatly enhancing the Raman scattering of PABT by multiple spectral resonances. Thus, a Raman switch concept by the Raman readout of PABT for the detection of TNT was proposed. The AgNPs paper at different sites exhibited a highly uniform sensitivity to TNT due to the layer-by-layer printing, and the sensitive limit could reach 1.6 × 10(-17) g/cm(2) TNT. Experimentally, upon applying a beam of near-infrared low-energy laser to slightly heat (but not destruct) TNT crystals, the resulting airborne TNT in the open environment was probed at the height of 5 cm, in which the concentration of airborne species was lower than 10 ppt by a theoretical analysis. Similarly, the odors from 1.4 ppm TNT in soil and 7.2, 2.9, and 5.7 ng/cm(2) TNT on clothing, leather, and envelope, respectively, were also quickly sensed for 2 s without destoying these inspected objects.

Self-assembled multilayer of alkyl graphene oxide for highly selective detection of copper(II) based on anodic stripping voltammetry

A novel graphene oxide multilayer with ordered structure was formed by self-assembly of alkyl functionalized graphene oxide on a metal substrate and showed specific electrochemical properties for the determination of copper(II) in solution using square wave anodic stripping voltammetry (SWASV). The ordered structure of the multilayer was evident from its regular X-ray patterns. Optimal electrochemical parameters, such as pH value, deposition potential and deposition time, were obtained for good SWASV performance. This alkyl chain functionalized graphene oxide film showed a high selectivity for the determination of Cu(II) in the presence of other metal ions. The detection limit of Cu(II) by the functional graphene oxide film was estimated to be 2.7 μM, which is below the permitted limit of 20 μM in drinking water given by the U.S. Environmental Protection Agency (EPA).

Molecularly imprinted polypyrrole nanonecklaces for detection of herbicide through molecular recognition-amplifying current response

Molecularly imprinted polypyrrole (PPy) nanonecklaces were facilely synthesized through a two-step oxidative polymerization route for the amperometric detection of non-electrochemically active herbicide. It has been demonstrated that dissolved oxygen can preoxidize pyrrole to form PPy oligomer bundles, which further self-assemble into necklace-like micelles in the presence of cetyltrimethylammonium bromide. Subsequently, these microstructures were immediately gelled through quick polymerization of residual pyrrole monomers, leading to the formation of PPy nanonecklaces. Meanwhile, herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was synchronously imprinted into the formed PPy and highly dense imprinted sites were generated in PPy nanonecklaces because the necklace-like structure with microgaps/pores provides the facile and complete removal of templates. The imprinted nanonecklaces exhibit the high capacity and fast kinetics to uptake 2,4-D molecules, and produce a imprinting factor of ~4.2. Importantly, the recognition and binding to 2,4-D significantly amplify the current response by a factor of 8 times in amperometric measurements, providing a sensitive detection of 2,4-D. The molecular imprinting strategy opens a novel avenue to the direct detection of non-electrochemically active species in a more convenient, simpler and cheaper way than the traditional competition-displacing approaches.

Molecularly Imprinted Shells from Polymer and Xerogel Matrices on Polystyrene Colloidal Spheres

We have devised a facile and general methodology for the synthesis of various molecularly imprinted shells at the surface of polystyrene (PS) colloidal spheres to recognize the explosive compound 2,4,6-trinitrotoluene (TNT). PS spheres with surface-functionalized carboxyl-group layers could direct a selective imprinting polymerization on their surface through the hydrogen-bonding interactions between surface carboxyl groups and amino monomers. Meanwhile, homogeneous polymerization in the solution phase was completely prevented by stepwise polymerization. The overall process led to the formation of monodisperse molecularly imprinted core-shell microspheres, and was very successful in the preparation of organic polymer and inorganic xerogel shells. Furthermore, greater capacity and faster binding kinetics towards target species were achieved, because surface-imprinted sites ensured the complete removal of templates, good accessibility to target molecules, and low mass-transfer resistance. The results reported herein, concerning the production of high-quality molecularly imprinted products, could also form the basis for the formulation of a new strategy for the fabrication of various functional coating layers on colloidal spheres with potential applications in the fields of separations and chemical sensing.

Atomic Oxygen Tailored Graphene Oxide Nanosheets Emissions for Multicolor Cellular Imaging

Graphene oxide (GO) has been widely used as a fluorescence quencher, but its luminescent properties, especially tailor-made controlling emission colors, have been seldom reported due to its heterogeneous structures. Herein, we demonstrated a novel chemical oxidative strategy to tune GO emissions from brown to cyan without changing excitation wavelength. The precise tuning is simply achieved by varying reaction times of GO nanosheets in piranha solution, but there is no need for complex chromatography separation procedures. With increasing reaction times, oxygen content on the lattice of GO nanosheets increased, accompanied by the diminution of their sizes and sp(2) conjugation system, resulting in an increase of emissive carbon cluster-like states. Thereby, the luminescent colors of GO were tuned from brown to yellow, green, and cyan, and its fluorescent quantum yields were enhanced. The obtained multicolored fluorescent GO nanosheets would open plenty of novel applications in cellular imaging and multiplex encoding analysis.