Renyong Liu

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.

Real-Time Discrimination and Versatile Profiling of Spontaneous Reactive Oxygen Species in Living Organisms with a Single Fluorescent Probe.

Fluorescent probes are powerful tools for the investigations of reactive oxygen species (ROS) in living organisms by visualization and imaging. However, the multiparallel assays of several ROS with multiple probes are often limited by the available number of spectrally nonoverlapping chromophores together with large invasive effects and discrepant biological locations. Meanwhile, the spontaneous ROS profilings in various living organs/tissues are also limited by the penetration capability of probes across different biological barriers and the stability in reactive in vivo environments. Here, we report a single fluorescent probe to achieve the effective discrimination and profiling of hydroxyl radicals (•OH) and hypochlorous acid (HClO) in living organisms. The probe is constructed by chemically grafting an additional five-membered heterocyclic ring and a lateral triethylene glycol chain to a fluorescein mother, which does not only turn off the fluorescence of fluorescein, but also create the dual reactive sites to ROS and the penetration capability in passing through various biological barriers. The reactions of probe with •OH and HClO simultaneously result in cyan and green emissions, respectively, providing the real-time discrimination and quantitative analysis of the two ROS in cellular mitochondria. Surprisingly, the accumulation of probes in the intestine and liver of a normal-state zebrafish and the transfer pathway from intestine-to-blood-to-organ/tissue-to-kidney-to-excretion clearly present the profiling of spontaneous •OH and HClO in these metabolic organs. In particular, the stress generation of •OH at the fresh wound of zebrafish is successfully visualized for the first time, in spite of its extremely short lifetime.

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.

Single clusters of self-assembled silver nanoparticles for surface-enhanced Raman scattering sensing of a dithiocarbamate fungicide

Hydrophobic silver (Ag) nanoparticles of ∼16 nm diameter were self-assembled as building blocks in an emulsion to form large spherical clusters upon the removal of organic solvents. The self-assembled clusters of Ag nanoparticles have diameters in the range 0.5–1.0 μm and are composed of thousands of densely packed Ag nanoparticles, leading to the generation of multiple active sites or hot spots for surface-enhanced Raman scattering (SERS) in a single cluster, as clearly observed using confocal Raman microscopy. Such single clusters of Ag nanoparticles show significant SERS activity for Rhodamine-6G and dithiocarbamates such as thiram. The enhancement factor for R6G was calculated to reach 1 × 109, which is possible for the observation of SERS signals of a single molecule of R6G according to literature reports. The as-prepared individual clusters of Ag nanoparticles have been demonstrated for the SERS detection of the agricultural chemical thiram. The results show that the detection limit for thiram is as low as 0.024 ppm, which is much lower than the maximal residue limit (MRL) of 7 ppm in fruit prescribed by U.S. Environmental Protection Agency (EPA). The system also possesses the ability to selectively detect dithiocarbamate compounds over other types of agricultural chemical. Furthermore, spiked and recovery tests show that the Ag nanoparticle clusters can be used to detect thiram in natural lake water and commercial apple juice without much interference.

Surface-enhanced Raman scattering sensor for theophylline determination by molecular imprinting on silver nanoparticles

A surface-enhanced Raman scattering (SERS)-based sensor for the determination of theophylline (THO) has been developed by imprinting the target molecules on the surface of silver nanoparticles. The desired recognition sites are generated after template removal and homogeneous distribution on the silver nanoparticles that have been incorporated within polymer matrix by the in situ reduction of theophylline-silver complexes, providing molecular recognition ability and SERS active surfaces. The theophylline molecules, complementary to the shape, size, and functionality of the recognition cavities, can selectively bind to the recognition sites at the surface of silver nanoparticles driven by the formation of hydrogen bonding and surface coordination. It has been demonstrated that the SERS signals of the theophylline molecules captured on the surface of the silver nanoparticles have a good reproducibility and a dose-response relationship to the target analytes, showing the potential for reliable identification and quantification of the bioactive compound. The molecular imprinting-based SERS sensor, like antibodies or enzymes, also possesses the ability to distinguish theophylline from the closely related structure caffeine due to the variations of molecular size and shape as well as the different affinity to silver ions.

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.

Label-free surface-enhanced Raman scattering imaging to monitor the metabolism of antitumor drug 6-mercaptopurine in living cells.

The molecular processes of drugs from cellular uptake to intracellular distribution as well as the intracellular interaction with the target molecule are critically important for the development of new antitumor drugs. In this work, we have successfully developed a label-free surface-enhanced Raman scattering (SERS) technique to monitor and visualize the metabolism of antitumor drug 6-mercaptopurine in living cells. It has been clearly demonstrated that Au@Ag NPs exhibit an excellent Raman enhancement effect to both 6-mercaptopurine and its metabolic product 6-mercaptopurine-ribose. Their different ways to absorb at the surface of Au@Ag NPs lead to the obvious spectral difference for distinguishing the antitumor drug and its metabolite by SERS spectra. The Au@Ag NPs can easily pass through cell membranes in a large amount and sensitively respond to the biological conversion of 6-mercaptopurine in tumor cells. The Raman imaging can visualize the real-time distribution of 6-mercaptopurine and its biotransformation with the concentrations in tumor cells. The SERS-based method reported here is simple and efficient for the assessments of drug efficacy and the understanding of the molecular therapeutic mechanism of antitumor drugs at the cellular level.

A chemically reactive Raman probe for ultrasensitively monitoring and imaging the in vivo generation of femtomolar oxidative species as induced by anti-tumor drugs in living cells

A chemically reactive Raman probe has been developed for ultrasensitively monitoring and imaging the in vivo generation of femtomolar oxidative species as induced by anti-tumor drugs in living cells.

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.