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Abstract
Detection of analyte in low concentrations has aroused research into the use of nanostructured electrodes to lower the detection limit and decrease response time. Limitations of low concentrations sensing are; the relatively few analyte present in solution (low signal) and a long diffusion of the analyte to the sensing surface (long response time). This thesis reports on low concentration detection for different species and the employment of gold nanoparticles or gold-coated magnetic nanoparticles (Au@MNPs) to achieve this.
Gold nanoparticles in solution were used in this work to improve both the detection limit and response time. To demonstrate this, particles were modified with L-cysteine and attached to an electrode surface as well as being dispersed into solution. These particles are used to detect copper in solution, requiring two cysteines to complex each copper ion. Hence the presence of copper ions, causes the particles to aggregate in solution and on the electrode surface. This aggregation collects large amounts of the analyte from solution as well as shortens diffusion time. A detection limit of 1 pM and response time of 10 minutes was observed. However, not all the nanoparticles aggregate in solution, thus limiting the amount of analyte detected. Consequently, Au@MNPs were used to further improve the detection limit and lower response time.
Same as gold nanoparticles, Au@MNPs can capture and concentrate the analyte, with added advantage that all the particles is collected and in a shorter period of time via a magnetic field. Aggregation of Au@MNPs modified with electroactive molecule over an electrode indicated that the lowest detected amount of 0.7 electroactive molecules per particle. Non-electroactive molecules, such as enrofloxacin were detected with resistance measurements of the Au@MNP films deposited across interdigitated electrodes. Change in resistance is related to distance between particles, which results from the antibodies specific to the analyte being present or absent after exposure to the analyte solution. A detection limit of 150 aM was observed with a detection time of 30 minutes with this Au@MNPs system.
The generality of the nanoparticle system could be applied to detect a wide range of biologically relevant molecules. A change in paradigm from having a sensing surface that is fixed in space to one that can thoroughly mix with the target analyte have solved many of the problems faced by low concentration sensing.