Single-walled Carbon Nanotube In Optical Sensing Applications

In recent years, with the development of nanoscience, nanotechnology has been integrated into biosensing. Carbon nanotubes (CNTs) are formed by the curly graphite layer. CNTs has the characteristics of graphite, such as heat resistance, corrosion resistance and electrical conductivity. It also has characteristics of nanomaterial, such as large surface area and high chemical activity. This paper would focus on the application of CNTs in optical biosensing, and build some new strategies of nanoscale optical biosensors.The paper includes two parts. The first part was Chapter 1, which reviews the researches of carbon nanotubes-based optical biosensing. The second part consists of other chapters, which is a research report.In Chapter 1, on the basis of a brief introduction of nanomaterials, the recent developments of carbon nantutobe-based optical analysis were emphatically reviewed.In Chapter 2, we report herein a label-free and sensitive fluorescent method for detection of thrombin using a G-quadruplex-based DNAzyme as the sensing platform. The thrombin-binding aptamer (TBA) is able to bind hemin to form the G-quadruplex-based DNAzyme, and thrombin can significantly enhance the activity of the G-quadruplex-based DNAzyme. The G-quadruplex-based DNAzyme is found to effectively catalyze the H2O2-mediated oxidation of thiamine, giving rise to fluorescence emission. This allows us to utilize the H2O2-thiamine fluorescent system for the quantitative analysis of thrombin. The assay shows a linear toward thrombin concentration in the range of 0.01-0.12 nM. The present limit of detection for thrombin is 1 pM, and the sensitivity for analyzing thrombin is improved by about 10000- fold as compared with the reported colorimetric counterpart. The work also demonstrates that thiamine is an excellent substrate for the fluorescence assay using the G-quadruplex-based DNAzyme as the sensing platform.In Chapter 3, the G-quadruplex-hemin DNAzyme has shown potential as a new catalytic label for the development of various biosensors. But the typical problem of high background limits the application of the method, which induced by excess hemin itself. In this work, we found that single-walled carbon nanotubes (SWNTs) can effectively decrease the background of G-quadruplex-hemin DNAzyme sensing platform. The method relies upon the strong affinity of SWNTs with hemin. In the presence of SWNTs, superfluous hemin is arrested the surface of SWNTs, and then is removed though centrifugation It was proved that SWNTs reduce the background of G-quadruplex-hemin DNAzyme sensing platform by ABTS-H2O2 colorimetric. Moreover, we detected K+ with the AGRO100 as the sensing element. The present limit of detection for K+ is 2 nM, and the sensitivity for analyzing K+ is improved by about 10-fold as compared with the reported colorimetric counterpart in the absence of SWNTs.In Chapter 4, this paper demonstrated a simple self-assembly protocol for synthesizing nanohybrid of hemin-SWNTs. Significantly, the hemin-SWNTs nanohybrid possesses the excellent peroxidase-like activity. It is found that peroxidase-like activity of this enzyme mimetic is significantly higher than that of the alone hemin. In the paper, the SWNTs don’t only play role of support, but also prevent hemin from the formation of m-oxo dimers or aggregation, help overcome the transport limitations and enhance the function and stability of the adsorbed hemin. Kinetic studies indicated that hemin-SWNTs hybrid material had even higher catalytic activity than the nature enzyme, HRP. The artificial enzyme was little influenced by pH and temperature. Based on these unique properties of the nanohybrid, we have developed a simple, cheap and sensitive colorimetric assay to detect glucose in blood sample. Otherwise the nanohybrid can be reused many times though centrifugation. In a word, it is expected that SWNTs-hemin nanohybrid could be applied in bioassay instead of HRP.In Chapter 5, a simple protocol for synthesizing nanohybrid of gold nanoparticles (AuNPs)-SWNTs was developed, which combines electroless deposition with dispersion of AuNPs on the functionalized SWNTs. It provided a challenge to synthesize AuNPs with high dispersion and small particle size on SWNTs. In the assay, we found the nanohybrid possesses predominant peroxidase-like activity, compared with other relative nanomaterials. As peroxidase mimics, the nanohybrid was used here for label-free colorimetric detection of DNA hybridization and cocaine with a detection limit of 2 nM based on the interaction of nanohybrid and DNA. This work will provide new insights into utilize SWNT peroxidase-like activity.In Chapter 6, we have developed a new multicolor fluorescent sensing system to detect multiple analytes in one pot. This design is based on the noncovalent assembly of dye-labeled aptamer with SWNTs byπ-stacking between the nucleotide bases and the SWNTs sidewalls. In the presence of the targets, the aptamer-target binding separates the assembly of dye-labeled aptamers and SWNTs, resulting in the restoration of fluorescence signal of the dye labeled with aptamers. As a proof of concept, we demonstrate that a two-color fluorescent system can simultaneously and selectively detect two targets (thrombin and adenosine triphosphate) in a single solution. Since the method is mix-and-detect manner, the present strategy is simple and cost-effective.In Chapter 7, an approach for visual sensing of ascorbic acid is presented. This method is based on the distance-dependent optical property of gold nanoparticles and the Cu+-catalyzed alkyne-azide click reaction. We prepared the terminal azide- and alkyne-functionalized gold nanoparticle probes. In the presence of Cu2+, ascorbic acid could rapidly induce the aggregation of the functionalized gold nanoparticles, thereby resulting in a red-to-purple (or pink) color change. Ascorbic acid can be quantified visually or using a UV-vis spectrometer. The present limit of detection for ascorbic acid is 3 nM. This method exhibits excellent selectivity over other common organic reducing compounds (such as glucose, cysteine, dopamine, thiamine and uric acid). Without the aid of any advanced instruments, the assay of ascorbic acid could be performed with one step at room temperature. This method provides a potentially useful tool for the on-site detection of ascorbic acid.