In the field of nanotechnology, nanosensors are instrumental for (a) monitoring physical and chemical phenomena in hard-to-reach areas, (b) detecting biochemicals in cellular organelles, and (c) detecting nanoscopic particles in industry and the environment. measure. ment. With continued progress in nanotechnology tools and increasing insight into the nanoscale phenomena, one can expect further advances in the field of nanosensors through improved performance of existing nanosensors and newer nanosensors based on novel mechanisms. He then joined the same university as a research associate and became one of the pioneers in Singapore in the processing and modeling of nanofibers using the electrospinning approach.
State Key Laboratory of Robotics Shenyang Institute of Automation Chinese Academy of Sciences Shenyang, China.
Introduction
Among the various types of CNTs, the most desirable are SWCNTs, which are available with diameters in the range of 1–3 nm and have lengths up to several tens of microns. Under special conditions, ultralong SWCNTs with lengths in the millimeter range were grown. In the following sections, synthesis approaches are discussed, followed by a discussion of various efforts to develop CNT-based sensors.
In the final section, we elaborate on the use of CNTs for drug delivery and bioimaging applications.
Synthesis of Carbon Nanotubes
Recent advances such as controlled CVD growth and success in growing 4 cm long defect-free CNTs [14] are a key step in this direction. Kinetics studies of growth further showed that the nucleation energy for such ultralong CNTs is about 2.8 eV [15], which is much higher than the diffusion energy of carbon atoms in bulk metals.
Relevant Physical Characteristics of Carbon Nanotubes
The CNTs have high mechanical strength [22,23] and in combination with light weight, they are desirable for lightweight composites [24] for high strength applications. Apart from these properties, nanotubes also have other properties such as field emission, nonlinear optical properties, electrochemical and novel photophysical properties, which can play a major role in the development of sensors and devices.
Chemical Sensors and MEMS-Based Nanotube Sensors
- Individual CNT Chemical Sensors
- CNT Network/Film-Based Chemical Sensors
- CNT Array-Based Gas Sensors
- Metal-Nanoparticle-Modified CNT Sensors
- Polymer-Functionalized CNT Chemical Sensors
- CNT-Templated Materials for Gas Sensors
- MEMS Sensors Using CNTs
26] found that the recovery time of FET sensors strongly depended on the voltage bias between source and drain. Most CNT-based sensors rely on the detection of a variety of electrical signals from CNTs such as change in resistance, capacitance, or resistance. Such a feature can be used to make ionization sensors, which can be fingerprinted. ionization characteristics of different gases.
The frequency response of the integrated sensor device is used to measure the atomic scale masses.
Biosensors, Drug Delivery, and Bioimaging
Biosensing Studies with Isolated CNTs
Upon physisorption or chemical interaction of proteins and other biomolecules, the FET devices show change in I-V characteristics [70-72]. Thus, CNT-FET devices can be used for sensing a range of analytes through appropriate surface functionalization. The CNTs showed modulation in NIR fluorescence intensity and this enabled the detection of glucose at 35 μM.
The CNTs in the substrate showed functional neural networks through the formation of synaptic connections.
Biosensing Using CNT Composites and Arrays
The authors showed that calcium imaging of NSCs displayed action potentials when current was applied through CNT substrates. These studies may influence the future applications of CNTs in the field of NSCs as potential stimulatory and replacement matrices and also as bio-implants. Electrochemical characterization has shown that CNTs in such electrodes have very useful properties, which can be used for chemical studies and biosensing [81-84].
Planarization techniques were followed to etch the top surface of the CNT/silica monoliths to expose the tips of the arrayed CNTs.
CNTs for Drug Delivery and Bioimaging Studies
These studies illustrate the vital use of CNTs for the targeted delivery and treatment of cancer. These studies show that the CNT emission can be used to track the flow of CNTs through cells and their interaction with cell receptor proteins can be monitored. In addition to the NIR emission and its use for bioimaging studies, Raman spectral signatures of CNTs can be used for studies of biological tissues.
The authors used Raman imaging of CNTs to study the effectiveness of CNTs in targeting tumor cells.
Conclusions and Outlook
The technique was proposed for the surface modification of microcantilevers (Yan et al.). The complexation kinetics can be followed by the analysis of the surface variation (in the case of the surface pressure under feedback control) or by the surface pressure variation (in the case of the position of the solid barriers) The linear response of the sensor to the urea was recorded for the concentration range of 0.3–1.4 mM.
Introduction
Fundamental Issues
- Localized Surface Plasmon Resonance of Noble
- Colloidal Stabilization
- Control of Nanoparticles Aggregation and Dispersion
- Quantification of Nanoparticle Aggregation and Dispersion
When individual particles are in close proximity or coalesce (e.g., the separation distance between the particles is comparable to or smaller than their radii), plasmon oscillations from neighboring particles can become coupled.10 A strong enhancement of the localized electric field within the interparticle spacing broadens and redshifts the spectra. SPR.10 For AuNPs, for example, gradually increased aggregation is characterized by a gradual decline of the plasmon peak at 520 nm and the appearance of a peak at ~600 nm (Figure 6.1). A, curves b–d), which are associated with a change in the color of the solution from red to dark red, purple, blue, etc. For AgNPs (e.g. 27 nm), aggregation is indicated as a decrease in intensity at 400 nm and the appearance of adsorption at ~500 nm (Figure 6.1B, curves b–d) and the color of the solution gradually changes from light yellow to orange depending on the degree of aggregation . The sensitivity of the colorimetric assay is determined by the molar extinction coefficients of the NP plasmon bands, which are determined by the material composition and particle size. 11,12 Larger NPs offer greater sensitivity because they have a higher molar extinction coefficient for their surface plasmon bands. 13–15 For AuNP 4 –35 nm, for example, the molar extinction coefficients increase by three orders of magnitude.15 The double logarithm of the extinction coefficient versus particle size in diameter shows a good linear relationship that can be expressed in equation 6.1, where ε is the extinction coefficient in M−1 cm−1 , D is the AuNP core diameter and k a = 10.80505.
Based on an arbitrary estimate, 90% (or more) of the colorimetric assays in the literature use spherical AuNPs. While most of the assays use either AuNPs or AgNPs separately, some assays have been inventive. Colloidal stabilization is a matter of introducing repulsive forces between particles to prevent colloids from coalescing.23 There are three mechanisms for colloid stabilization, namely electrostatic stabilization (Figure 6.2A), steric stabilization (Figure 6.2B) and electrosteric stabilization (Figure 6.2 A). 6.2C) which are usually provided by charged small molecules, polymers and electrolytes, respectively.
For spherical AuNPs and AgNPs, electrostatic stabilization is usually achieved by means of a coating of citrate ions formed during particle formation by means of the classic citrate reduction reactions.24 The surface charges, together with the counterions in solution, form a repulsive electrical double layer that stabilizes the particles against van der Waals' attractive forces.25 Since the thickness of the electric double layer is determined by the bulk ionic strength of the liquid medium, the electrostatic stabilization is very sensitive to salt concentration. As shown in Figure 6.1, progressive particle aggregation is recorded as a gradual spectrum shift, i.e. a gradual increase in intensity at the longer wavelengths that are representative of aggregated particles and a decrease in intensity at the original wavelength for scattered particles. The ratio of absorbance at a longer wavelength to original wavelength (eg, A600/A520 for AuNPs and A500/A400 for AgNPs) at a given time is a quantitative measure of aggregation and dispersion status.
Sometimes the variation of the integrated absorbance between two selected wavelengths is used for quantitative analysis.32–34. In some of the type I cross-linking aggregation-based assays (for details, see Section 6.3), separation or dissociation of NPs aggregates, accompanied by a blue-to-red color change for AuNPs and orange-to-yellow change for AgNP 'er , is the target of a particular biological process (e.g. DNA melting, 8,16,20 binding of DNA binders,35,36 and DNA cleavage37).
Colorimetric Assays for Various Analyte Species and Biological Processes
- Nucleic Acids
- Aptamers and Their Targets
- DNA Binders—Drug, Metal Ion, and Protein
- Enzymatic Phosphorylation and Dephosphorylation
- Enzymatic Cleavage of Nucleic Acids
- DNA Cleavage by Endonucleases
- DNAzyme Cleavage for Metal Sensing
In the aforementioned assays, the preparation of DNA-NP or PNA-NP conjugates with well-controlled surface coverage and DNA/PNA stability is a critical step. Li and Rothberg41,42 found that single-stranded DNA (ssDNA) can be absorbed on AuNPs coated with citrate ions. In contrast, dsDNA has limited affinity for AuNPs and therefore lacks protection for the particles (Figure 6.4, Scheme C).
The essential difference arises because ssDNA can be unwound enough to expose its bases, whereas dsDNA has a stable double-helix geometry that always presents a negatively charged phosphate backbone.41,42 Using the distinct electrostatic properties of ssDNA and dsDNA, Li and Rothberg, and others have developed assays to detect specific DNA sequences with single base mismatch sensitivity in both synthetic oligonucleotides41,43,44 and genomic DNA after PCR amplification.42, 45 In type II assays, elimination of DNA-conjugation of AuNPs not only simplifies assay preparation, but also leads to a faster signal response because DNA-DNA hybridization on the particles is not needed. The finding that ssDNA, but not dsNDA (or structured DNA), can protect AuNPs against salt-induced aggregation has been the underlying principle of many Type II assays for non-DNA-bound analytes (see sections and 6.3.5.2). My group has recently developed a type II DNA assay using charge-neutral PNAs as probes (Figure 6.4, Scheme D).
Unlike Scheme C, which uses differential electrostatic properties of ssDNA and dsDNA to check the salt stability of NPs, our assay exploits the different charge effects of PNA (neutral) and PNA-DNA (negative) complex on the intrinsic stability of metal NPs. 19 We found that charge-neutral PNA oligomers (10–22 more of the tested samples) are effective coagulants of citrate ion-coated AuNPs and AgNPs. However, when PNA probes hybridize to specific DNA, aggregation can be largely slowed down in a DNA concentration and sequence dependent manner. By measuring the intrinsic stability of AuNPs and AgNPs (no salt is added), specific DNA sequences can be detected with sensitivity to a single base mismatch.
Interestingly, we have found that the PNA-DNA complex, despite the presence of dsDNA-like double helical geometry, can effectively protect NPs, even better than ssDNA. For assays using the same materials (eg, AuNPs) of similar size, cross-linking aggregation assays (Scheme A1) provide a higher sensitivity compared to non-cross-linking assays (Schemes B to D).
