Zhixin Zhou

Dissolution and Liquid Crystals Phase of 2D Polymeric Carbon Nitride

Graphite-phase polymeric carbon nitride (GPPCN) has emerged as a promising metal-free material toward optoelectronics and (photo)catalysis. However, the insolubility of GPPCN remains one of the biggest impediments toward its potential applications. Herein, we report that GPPCN could be dissolved in concentrated sulfuric acid, the first feasible solvent so far, due to the synergistic protonation and intercalation. The concentration was up to 300 mg/mL, thousands of time higher than previous reported dispersions. As a result, the first successful liquid-state NMR spectra of GPPCN were obtained, which provides a more feasible method to reveal the finer structure of GPPCN. Moreover, at high concentration, a liquid crystal phase for the carbon nitride family was first observed. The successful dissolution of GPPCN and the formation of highly anisotropic mesophases would greatly pave the potential applications such as GPPCN-based nanocomposites or assembly of marcroscopic, ordered materials.

Chemical Cleavage of Layered Carbon Nitride with Enhanced Photoluminescent Performances and Photoconduction.

Graphene quantum dots (GQDs) and carbon dots (C-dots) have various alluring properties and potential applications, but they are often limited by unsatisfied optical performance such as low quantum yield, ambiguous fluorescence emission mechanism, and narrow emission wavelength. Herein, we report that bulk polymeric carbon nitride could be utilized as a layered precursor to prepare carbon nitride nanostructures such as nanorods, nanoleaves and quantum dots by chemical tailoring. As doped carbon materials, these carbon nitride nanostructures not only intrinsically emitted UV lights but also well inherited the explicit photoluminescence mechanism of the bulk pristine precursor, both of which were rarely reported for GQDs and C-dots. Especially, carbon nitride quantum dots (CNQDs) had a photoluminescence quantum yield (QY) up to 46%, among the highest QY for metal-free quantum dots so far. As examples, the CNQDs were utilized as a photoluminescence probe for rapid detection of Fe(3+) with a detection limit of 1 μM in 2 min and a photoconductor in an all-solid-state device. This work would open up an avenue for doped nanocarbon in developing photoelectrical devices and sensors.

Environment-friendly preparation of porous graphite-phase polymeric carbon nitride using calcium carbonate as templates, and enhanced photoelectrochemical activity

Graphite-phase polymeric carbon nitride (GPPCN) is one kind of new organic semiconductor for photoelectric conversion, photocatalysis and other important catalytic reactions. However, the low surface area of bulk GPPCN limits its potential applications. Here, we report the preparation of porous GPPCN using industrially available calcium carbonate particles as the hard template; these are not only low-cost, but also easily removed by dilute hydrochloric acid. Interestingly, upon engineering the w/w ratio of template to GPPCN precursor along with the template sizes, our approach resulted in increases of about 4 and 7.5 times in the cathodic photocurrent under visible light (λ > 420 nm) irradiation compared with bulk GPPCN when biased at −0.2 V and 0 V (vs. Ag/AgCl), respectively. These photoelectrochemical activities were higher than those of porous GPPCN obtained by all other reported techniques including the common strategy of using silica nanoparticle templates. This study opens a new avenue to explore the fascinating GPPCN materials for solar energy conversion and environmental remediation, especially for large-scale industrial applications.

Electrochemiluminescence resonance energy transfer between graphene quantum dots and gold nanoparticles for DNA damage detection

Bright blue luminescent graphene quantum dots (GQDs) with major graphitic structured nanocrystals and a photoluminescence (PL) quantum yield of 15.5% were synthesized and used to monitor DNA damage. The GQDs were prepared by ultraviolet irradiation without using a chemical agent. The as-prepared GQDs showed excitation-dependent PL and stable electrochemiluminescence (ECL) behaviors. Gold nanoparticles (AuNPs) were linked with a probe of single-stranded DNA (cp53 ssDNA) to form AuNPs-ssDNA. The ECL signal of the GQDs could be quenched by non-covalent binding of the AuNPs-ssDNA to the GQDs, due to the occurrence of an electrochemiluminescence resonance energy transfer between the GQDs and the AuNPs. When AuNPs-ssDNA was then hybridized with target p53 DNA to form AuNPs-dsDNA, the non-covalent interaction between the GQDs and the ds-DNA weakened and the ECL of the GQDs recovered. This engendered an ECL sensor for the detection of target p53 ssDNA, with a detection limit of 13 nM. The resultant ECL sensor could be used for DNA damage detection based on its different bonding ability to damaged target p53 ssDNA and cp53 ssDNA linked AuNPs. The presented method could be expanded to the development of other ECL biosensors, for the quantification of nucleic acids, single nucleotide polymorphisms or other aptamer-specific biomolecules.

Chemically Modulated Carbon Nitride Nanosheets for Highly Selective Electrochemiluminescent Detection of Multiple Metal-ions

Chemical structures of two-dimensional (2D) nanosheet can effectively control the properties thus guiding their applications. Herein, we demonstrate that carbon nitride nanosheets (CNNS) with tunable chemical structures can be obtained by exfoliating facile accessible bulk carbon nitride (CN) of different polymerization degree. Interestingly, the electrochemiluminescence (ECL) properties of as-prepared CNNS were significantly modulated. As a result, unusual changes for different CNNS in quenching of ECL because of inner filter effect/electron transfer and enhancement of ECL owing to catalytic effect were observed by adding different metal ions. On the basis of this, by using various CNNS, highly selective ECL sensors for rapid detecting multiple metal-ions such as Cu(2+), Ni(2+), and Cd(2+) were successfully developed without any labeling and masking reagents. Multiple competitive mechanisms were further revealed to account for such enhanced selectivity in the proposed ECL sensors. The strategy of preparing CNNS with tunable chemical structures that facilely modulated the optical properties would open a vista to explore 2D carbon-rich materials for developing a wide range of applications such as sensors with enhanced performances.

Reversible Assembly of Graphitic Carbon Nitride 3D Network for Highly Selective Dyes Absorption and Regeneration

Responsive assembly of 2D materials is of great interest for a range of applications. In this work, interfacial functionalized carbon nitride (CN) nanofibers were synthesized by hydrolyzing bulk CN in sodium hydroxide solution. The reversible assemble and disassemble behavior of the as-prepared CN nanofibers was investigated by using CO2 as a trigger to form a hydrogel network at first. Compared to the most widespread absorbent materials such as active carbon, graphene and previously reported supramolecular gel, the proposed CN hydrogel not only exhibited a competitive absorbing capacity (maximum absorbing capacity of methylene blue up to 402 mg/g) but also overcame the typical deficiencies such as poor selectivity and high energy-consuming regeneration. This work would provide a strategy to construct a 3D CN network and open an avenue for developing smart assembly for potential applications ranging from environment to selective extraction.

Potential-Modulated Electrochemiluminescence of Carbon Nitride Nanosheets for Dual-Signal Sensing of Metal Ions

As an emerging semiconductor, graphite-phase polymeric carbon nitride (GPPCN) has drawn much attention not only in photocatalysis but also in optical sensors such as electrochemiluminescence (ECL) sensing of metal ions. However, when the concentrations of interfering metal ions are several times higher than that of the target metal ion, it is almost impossible to distinguish which metal ion changes the ECL signals in real sample detection. Herein, we report that the dual-ECL signals could be actuated by different ECL reactions merely from GPPCN nanosheets at anodic and cathodic potentials, respectively. Interestingly, the different metal ions exhibited distinct quenching/enhancement of the ECL signal at different driven potentials, presumably ascribed to the diversity of energy-level matches between the metal ions and GPPCN nanosheets and catalytic interactions of the intermediate species in ECL reactions. On this basis, without any labeling and masking reagents, the accuracy and reliability of sensors based on the ECL of GPPCN nanosheets toward metal ions were largely improved; thus, the false-positive result caused by interferential metal ions could be effectively avoided. As an example, the proposed GPPCN ECL sensor with a detection limit of 1.13 nM was successfully applied for the detection of trace Ni(2+) ion in tap and lake water.

DNA-responsive disassembly of AuNP aggregates: influence of nonbase-paired regions and colorimetric DNA detection by exonuclease III aided amplification†

Due to great potential in nanobiotechnology, nanomachines, and smart materials, DNA-directed disassembly of gold nanoparticles (AuNPs) has been extensively explored. In a typical system, nonbase-paired regions (e.g., overhangs and gaps in the linker DNA and oligonucleotide spacers between thiol group and hybridization sequence) are indispensable portions in the disassembly of AuNPs based on DNA displacement reaction. Therefore, it is necessary to study the effect of nonbase-paired regions to improve the kinetics of disassembly of AuNPs. Herein, the disassembly rate of AuNPs based on DNA displacement reaction was investigated by using different length spacers and linker DNA containing various lengths of gaps or overhangs. Interestingly, it was revealed that among the gaps in the linker DNA could be most effectively used to improve the disassembly rate of the AuNPs. As a result, when we introduced gaps into linker DNA, the DNA displacement reaction of AuNPs was markedly shortened to less than 50 min, which was much faster than the previous methods. As a proof of the importance of our findings, a rapid AuNP-based colorimetric DNA biosensor has been successfully prepared. In addition, we showed that the signal of the biosensors could be further amplified using exonuclease III, resulting in a much lower detection limit in comparison with previous sensors similarly using AuNP aggregates as probes.

Synthesis of B-doped hollow carbon spheres as efficient non-metal catalyst for oxygen reduction reaction

The oxygen reduction reaction (ORR) is one of the crucial reactions in fuel cells and metal–air batteries. Heteroatom doped carbon spheres can serve as alternative low-cost non-metal electrocatalysts for ORR. Herein, we developed an effective route to the synthesis of uniform and electrochemically active B-doped hollow carbon nanospheres (BHCSs). BHCSs were synthesized via the carbonization of a boric phenolic resin supported by SiO2, followed by etching the SiO2 template. The content of B, B dopant species and specific surface area were adjusted by changing the content of the B precursor and the calcination temperature. Moreover, their influence on the performance of electrocatalytic activity was explored. It was found that, among these B-doping type materials (BC2O, BCO2, B4C and BC3), B–C bonds (B4C and BC3) played a crucial role on improving the electrocatalytic activity. Compared with the hollow carbon nanospheres (HCSs), a 70 mV positive shift of the onset potential and 1.7 times kinetic current density could be clearly observed with BHCSs. In addition, the BHCSs revealed better stability and methanol tolerance than commercial Pt/C (HiSPEC™ 3000, 20%). Thus, the as-prepared BHCSs, as inexpensive and efficient non-metal ORR catalysts, may have a promising application in direct methanol fuel cells.

Comparison Study of the Photoelectrochemical Activity of Carbon Nitride with Different Photoelectrode Configurations

Polymeric carbon nitride (CN) has recently emerged as a novel metal-free semiconductor due to its unique electronic structure, wide availability, and promising applications in photoelectrochemical solar energy conversion. However, few works regarding CN photoelectrode optimization such as by minimization of unwanted grain boundary effects have been reported, which would greatly influence the photoelectrochemcial conversion efficiency. Herein, three general ways of preparing CN photoelectrode are presented and compared, including drop-casting of CN particles, or further blendeding with Nafion or PEDOT-PSS as the binder. In addition, the influences of CN particle sizes (0.5, 1.1, and 3.2 μm) and the film thickness (i.e., the loading amount) to the overall photoelectrochemcial activity were also evaluated in detail. As a result, when PEDOT-PSS acted as binder, CN particles with size of 0.5 μm and an optimal loading amount (2.4 mg/cm(2)) were adopted; the as-prepared CN photoelectrode had much superior photoelectrochemical activity than all other counterparts. Therefore, this study would pave the way for preparing CN photoelectrode of superior quality so as to promote CN materials to be better applied in solar fuel and sensing applications.