Hongliang Tan

Electrochemical Sensing and Biosensing Platform Based on Biomass-Derived Macroporous Carbon Materials

A three-dimensional (3D) macroporous carbon (3D-KSCs) derived from kenaf stem (KS) is proposed as a novel supporting material for electrochemical sensing and a biosensing platform. A series of 3D-KSCs/inorganic nanocomposites such as Prussian blue (PB) nanoparticles (NPs)-carboxylic group-functionalized 3D-KSCs (PBNPs-3D-FKSCs), CuNiNPs-3D-KSCs, and CoNPs-3D-KSCs were prepared by a facile two-step route consisting of carbonization and subsequent chemical synthesis or one-step carbonization of KS-metal ion complex. The obtained 3D-KSCs/inorganic nanocomposites were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscopy, and Fourier transform-infrared spectroscopy. A whole piece of 3D-KSCs/nanocomposites was used to prepare an integrated 3D-KSCs/nanocomposite electrode. Compared to the electrode modified by graphene, carbon nanotubes and their derivatives, which can form close-packed structure after assembled on electrode surface, the integrated 3D-KSCs/nanocomposite electrode shows a 3D honeycomb porous structure. Such structure provides a large specific surface area, effectively supports a large number of electro-active species, and greatly enhances the mass and electron transfer. The electrochemical behaviors and electrocatalytic performances of the integrated 3D-KSCs/inorganic nanocomposite electrode were evaluated by cyclic voltammetry and the amperometric method. The resulted PBNPs-3D-FKSCs, CuNiNPs-3D-KSCs, and CoNPs-3D-KSCs electrode show good electrocatalytic performances toward the reduction of H2O2, the oxidation of glucose and amino acid, respectively. Therefore, the low-cost, renewable, and environmentally friendly 3D-KSCs should be promising supporting materials for an electrochemical sensor and biosensor.

pH-Switchable Electrochemical Sensing Platform based on Chitosan-Reduced Graphene Oxide/Concanavalin A Layer for Assay of Glucose and Urea

A facile and effective electrochemical sensing platform for the detection of glucose and urea in one sample without separation was developed using chitosan-reduced graphene oxide (CS-rGO)/concanavalin A (Con A) as a sensing layer. The CS-rGO/Con A with pH-dependent surface net charges exhibited pH-switchable response to negatively charged Fe(CN)6(3-). The principle for glucose and urea detection was essentially based on in situ pH-switchable enzyme-catalyzed reaction in which the oxidation of glucose catalyzed by glucose oxidase or the hydrolyzation of urea catalyzed by urease resulted in a pH change of electrolyte solution to give different electrochemical responses toward Fe(CN)6(3-). It was verified by cyclic voltammograms, differential pulse voltammograms, and electrochemical impedance spectroscopy. The resistance to charge transfer or amperometric current changed proportionally toward glucose concentration from 1.0 to 10.0 mM and urea concentration from 1.0 to 7.0 mM. On the basis of human serum experiments, the sensing platform was proved to be suitable for simultaneous assay of glucose and urea in a practical biosystem. This work not only gives a way to detect glucose and urea in one sample without separation but also provides a potential strategy for the detection of nonelectroactive species based on the enzyme-catalyzed reaction and pH-switchable biosensor.

Determination of tetracycline in milk by using nucleotide/lanthanide coordination polymer-based ternary complex

The meta-organic coordination polymers have been emerged as fascinating nanomaterials because of their tunable nature. In this work, we employed lanthanide coordination polymer self-assembled from adenosine monophosphate (AMP) and europium ion (Eu(3+)) as receptor reagent and citrate (Cit) as ancillary ligand to construct a fluorescent sensor for the detection of tetracycline (Tc) in milk. The co-coordination of Cit and Tc with Eu(3+) on the surface of the coordination polymer AMP/Eu leads to the formation of ternary complex which emitted strong fluorescence due to the removal of coordinated water molecules and an intramolecular energy transfer from Tc to Eu(3+). The fluorescent intensity of Eu(3+) displayed a good linear response to Tc concentrations in the range of 0.1-20 μM with a detection limit of 60 nM. This method was successfully applied to determine the levels of Tc in milk, which is the first application of coordination polymer as a fluorescent sensor in real sample. Compared with other Eu(3+)-based fluorescent methods for Tc detection, the presented method allows simple, direct analysis of Tc without requiring special reaction media or complicated prepreparation processes. This straightforward strategy could be extended to the preparation of other lanthanide coordination polymer-based fluorescent probes for applications in biosensing, imaging, drug delivery, and so on.

Metal organic framework-derived anthill-like Cu@carbon nanocomposites for nonenzymatic glucose sensor

A novel nonenzymatic glucose sensor was constructed based on anthill-like Cu@carbon nanocomposites which were derived from a Cu-based metal organic framework by a simple thermolysis method. The final nanocomposites were characterized by scanning electron microscopy, thermogravimetric analysis, X-ray powder diffraction and electrochemical techniques. The results showed that the derived nanocomposites maintained the morphology of the original materials upon thermolysis, while the produced Cu nanoclusters were embedded in three-dimensional carbon frameworks and presented an anthill-like structure. Since the final products gave a sufficiently large specific surface area, good catalytic activity towards the oxidation of glucose and appropriate pores for electrolyte transfer, the resultant glucose sensor based on the anthill-like Cu@carbon nanocomposites showed a wide linear range of 0.2–8.0 mM and a low detection limit of 29.8 μM. The low cost, simple preparation and good catalytic activity of anthill-like Cu@carbon nanocomposites render them promising candidates as electrode materials for the construction of novel nonenzymatic sensors.

A sensitive fluorescent assay for thiamine based on metal-organic frameworks with intrinsic peroxidase-like activity

Metal-organic frameworks (MOFs) with tunable structures and properties have recently been emerged as very interesting functional materials. However, the catalytic properties of MOFs as enzymatic mimics remain to be further investigated. In this work, we for the first time demonstrated the peroxidase-like activity of copper-based MOFs (HKUST-1) by employing thiamine (TH) as a peroxidase substrate. In the presence of H2O2, HKUST-1 can catalyze efficiently the conversion of non-fluorescent TH to strong fluorescent thiochrome. The catalytic activity of HKUST-1 is highly dependent on the temperature, pH and H2O2 concentrations. As a peroxidase mimic, HKUST-1 not only has the features of low cost, high stability and easy preparation, but also follows Michaelis-Menten behaviors and shows stronger affinity to TH than horseradish peroxidase (HRP). Based on the peroxidase-like activity of HKUST-1, a simple and sensitive fluorescent method for TH detection has been developed. As low as 1 μM TH can be detected with a linear range from 4 to 700 μM. The detection limit for TH is about 50 fold lower than that of HRP-based fluorescent assay. The proposed method was successfully applied to detect TH in tablets and urine samples and showed a satisfactory result. We believed that the present work could improve the understanding of catalytic behaviors of MOFs as enzymatic mimics and find out a wider application in bioanalysis.

A Green Strategy to Prepare Metal Oxide Superstructure from Metal-Organic Frameworks

Metal or metal oxides with diverse superstructures have become one of the most promising functional materials in sensor, catalysis, energy conversion, etc. In this work, a novel metal-organic frameworks (MOFs)-directed method to prepare metal or metal oxide superstructure was proposed. In this strategy, nodes (metal ions) in MOFs as precursors to form ordered building blocks which are spatially separated by organic linkers were transformed into metal oxide micro/nanostructure by a green method. Two kinds of Cu-MOFs which could reciprocally transform by changing solvent were prepared as a model to test the method. Two kinds of novel CuO with three-dimensional (3D) urchin-like and 3D rods-like superstructures composed of nanoparticles, nanowires and nanosheets were both obtained by immersing the corresponding Cu-MOFs into a NaOH solution. Based on the as-formed CuO superstructures, a novel and sensitive nonenzymatic glucose sensor was developed. The small size, hierarchical superstructures and large surface area of the resulted CuO superstructures eventually contribute to good electrocatalytic activity of the prepared sensor towards the oxidation of glucose. The proposed method of hierarchical superstructures preparation is simple, efficient, cheap and easy to mass production, which is obviously superior to pyrolysis. It might open up a new way for hierarchical superstructures preparation.

Terbium-Based Coordination Polymer Nanoparticles for Detection of Ciprofloxacin in Tablets and Biological Fluids

The metal-organic coordination polymers with tunable structures and properties have been rapidly emerging as very important functional materials. In this work, we prepared terbium (Tb(3+))-based coordination polymer nanoparticles (CPNPs) by employing adenine (Ad) as bridging ligands. The CPNPs was further used as a receptor reagent for ciprofloxacin (CF) detection in aqueous solution. Addition of CF induces a typical emission of Tb(3+) due to the formation of Ad/Tb-CF complex and the sensitization of CF. The fluorescent intensity of Tb(3+) was enhanced linearly with increasing the CF concentration from 60 nM to 14 μM. The detection limit for CF in aqueous solution is 60 nM. The Ad/Tb CPNPs was successfully applied to detect CF in tablet and urine samples and showed a satisfactory result. Compared with other methods, the proposed method is advantageous because that it provides a very simple strategy for CF detection, which does not require complicated sample pretreatment processes or special reaction media. The proposed strategy could be contributed to expand the potential applications of lanthanide coordination polymers in biological and environmental fields.

A novel nonenzymatic hydrogen peroxide sensor based on three-dimensional porous Ni foam modified with a Pt electrocatalyst

A novel nonenzymatic hydrogen peroxide (H2O2) sensor was simply prepared by depositing Pt nanoparticles (Pt NPs) onto Ni foam using UV-irradiation. Scanning electron microscopy was applied to characterize the changes of morphologies with UV-irradiation time. Energy dispersive spectroscopy confirmed that the Pt NP–Ni foam was mainly composed of Pt and Ni. The Pt NP–Ni foam electrode shared the unique advantages of Pt NPs (such as the good electrocatalytic activity) and Ni foam (such as the high electric conductivity, large surface area and high porosity). Its application in H2O2 detection, surprisingly, showed the high sensitivity and low detection limit. The linear range was from 0.005 to 0.85 mM. The sensitivity was 829 μA cm−2 mM−1 and the detection limit was 0.3 μM (S/N = 3). The H2O2 sensor also showed long-term stability. Therefore, the sensor is more suitable for the detection of H2O2 concentration.

Lanthanide based dual-emission fluorescent probe for detection of mercury (II) in milk

It is highly desirable to develop a simple and sensitive method for Hg(2+) detection because of the dangerous nature of Hg(2+). In this work, we prepared a dual-emission fluorescent probe for Hg(2+) detection by combining two lanthanide chelates with different emission wavelengths. Green-emitting terbium (Tb(3+)) chelates as reference signals were embedded into SiO2 nanoparticles and red-emitting europium (Eu(3+)) chelates as response units were covalently linked to the surface of silica shell. Upon the addition of Hg(2+), the fluorescence of Eu(3+) chelates can be selectively quenched, while the fluorescence of Tb(3+) chelates remained unchanged. As a kind of Hg(2+) nanosensor, the dual-emission fluorescent probe exhibited excellent selectivity to Hg(2+) and high sensitivity up to 7.07 nM detection limit. The Hg(2+) levels in drinking water and milk samples were determined by using the dual-emission fluorescent probe with satisfied recovery. Additionally, our probe has a long enough fluorescence lifetime, which can avoid the interference from autofluorescence of the biological samples. We envision that the proposed probe could find great potential applications for ultrasensitive time-resolved fluorometric assays and biomedical imaging in the future.

A turn on fluorescent sensor based on lanthanide coordination polymer nanoparticles for the detection of mercury(II) in biological fluids

Coordination polymers have recently emerged as very fascinating materials due to their tunable nature. In this work, we develop a lanthanide coordination polymer (CPNP)-based turn on sensor for Hg2+ detection by employing the strategy of inner filter effect (IFE). This kind of CPNP is composed of europium ions (Eu3+) as metal centers and isophthalic acid (IPA) as bridging ligands that can sensitize the fluorescence of Eu3+. Because the excitation spectrum of Eu/IPA CPNPs greatly overlaps with the absorbance band of imidazole-4,5-dicarboxylic acid (Im), the presence of Im can significantly quench the fluorescence of Eu/IPA CPNPs through a process of IFE. Upon the addition of Hg2+, however, the Im-quenched fluorescence of Eu/IPA CPNPs can be recovered due to the suppression of the IFE of Im through the formation of a Hg/Im complex. As a fluorescent sensor for Hg2+ detection, Eu/IPA CPNPs not only show high sensitivity up to a detection limit of 2 nM and excellent selectivity, but also possess the advantages of fast response, simple preparation procedure and flexible sensing performance. More importantly, interference from the background fluorescence of biological fluids can be efficiently eliminated via a time-resolved detection mode. The presence of the sensing strategy would be beneficial to the design of other lanthanide coordination polymer-based fluorescent sensors.