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    • br Conclusion The parameters of the shutter perforation widt

    2018-10-30


    Conclusion The parameters of the shutter perforation width S, shutter thickness t, sense electrode width E, and shutter to electrode gap g were studied in relation to each other and with respect to the total shutter perforation repetition period L. It is found that shutter thickness and gap spacing between the underlying electrodes should be minimized as much as possible as they very strongly affect the MEFM signal. Exploration of S and E shows that the best MEFM design is one where shutter perforations are approximately 3.5× the size of the sense electrodes.
    Acknowledgments This research was financially supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada, and by the Manitoba Hydro of Winnipeg, Canada.
    Introduction In recent years, Surface Enhanced Raman Spectroscopy (SERS) has emerged as a very important technique for the ultra-sensitive detection of bio-molecules [1]. This technique combines the advantages of the Raman Effect such as the high specificity (ability to identify a given molecular species in the presence of many other chemicals) with nano-plasmonics for Raman signal enhancement. This makes it a very selective and sensitive method for quantitative detection of molecules down to the single molecule level [2]. We have carried out experiments to establish the efficacy of the SERS technique for quantitative detection of Rhodamine 6G and Crystal Violet [3,4] down to molar concentrations of 10M. Recently we have applied our experimental techniques for the quantitative determination of standard Wortmannin [5]. In this work we apply the SERS technique for the quantitative detection of glucose so that this method can be employed for diagnostic applications in diabetes. Diabetes mellitus is a metabolic disorder and considered as a major problem which affects over 380million people across the world and about 3.2million people die annually due to this ailment [6]. It is estimated that people living with diabetes will increase to 55% by 2035. In order to measure the blood glucose levels we need to draw blood from patients and this is a painful procedure. However, studies have also detected presence of glucose in the urine and saliva samples of diabetic patients. The usual molar concentration levels of glucose in urine are in the range 0–0.8mM, and higher glucose levels in blood lead to higher glucose levels in urine. The glucose levels in saliva are at much lower levels in the range 50μM–500μM but given the sensitivity of the SERS technique are measurable. So there is an urgent need to develop non-invasive or minimally invasive methods for frequent glucose monitoring. Accurate quantitative detection and evaluation of glucose has not been possible due to lack of a sensitive and molecule specific method. We show here that it is possible in principle to detect quantitatively glucose levels in aqueous glucose solutions at levels beyond the physiological concentration levels in blood. SERS detection of glucose is limited by two factors. One factor is the low Raman scattering cross-section of the glucose molecule [7] and the second one is the poor affinity of glucose molecules to be adsorbed on metal surfaces [8,9]. Many approaches have been taken to address the aforementioned challenges. Van Duyne\'s group [10] came up with the idea to modify the metal surface chemically with alkane thiol so that the alkane molecules sticking out of the metal surface form a partition layer that traps the glucose molecule close to the metal surface. Mixed decanethiol/mercaptohexanol partition layers were also investigated [11]. Other SERS substrates have also been studied for glucose detection [12–14]. In these studies the thiol containing molecule was used to attach the molecule to the metal surface but the trapping of the glucose near the metal surface is not specific to glucose. Torul et al. reported a paper-membrane based SERS platform [15] and also two component self-assembled monolayer functionalized substrates [16] for glucose detection. In these studies they were able to detect glucose concentration up to 5mM and 0.5mM respectively. Kong et al. [17] used tri-osmium carbonyl cluster-boronic acid in SERS based assay. They used CO stretching vibrations (1800cm to 2200cm) of the metal carbonyl for quantification. The limit of detection (LOD) was 0.1mM. Zhong et al. [18] reported d-glucose detection in the physiological concentration range of 0.9mM to 30mM. Au nanoparticle coated zinc oxide nanowires and colloidal Ag nanoparticles were used as SERS substrates and 3,3ʹ-boronic benzyl viologen (BBV) as Raman probe molecule. The LOD was 0.25mM. But for early detection of diabetes it would be desirable to detect glucose levels present in saliva at considerably lower levels and the method proposed in the present work is able to detect glucose to such levels.