# Advancements in Glucose Monitoring: From Traditional Methods to Wearable Sensors
## 3 Minimally Invasive Methods
### Raman Spectroscopy  
Raman spectroscopy is extensively used in biomolecule detection due to its high selectivity towards the target molecule. This technology has previously been applied to analyze various biological samples, including glucose levels, pH, and redox potentials [12; 13; 14]. However, the direct detection of biochemicals using SERS remained challenging due to poor adsorption. To improve Raman signal intensity, SERS sensors based on various nanostructures, such as silver or gold nanopillars, have been used, significantly enhancing weak Raman signals by factors ranging from \(10^{6}\) to \(10^{8}\)[15; 16; 17]. [PERSON] et al. developed innovative implantable nanospheres incorporating a silver film and self-assembled monolayers (SAMs). The SAMs, applied as functional layers on the silver film, enhance the analyte signal and are utilized in vivo for SERS-based applications [70]. The silver nanoparticles coated on the microneedles enhance Raman signals, while the 1-decanethiol coating serves as a glucose-capturing agent, concentrating glucose molecules in the interstitial fluid (ISF) (Figure 6a) [71]. On the other hand, [PERSON] et al. developed, 4-Mercaptophenylboronic acid (4-MPBA)-functionalized gold (Au)-coated zinc oxide (ZnO) nanowires. This SERS substrate was later used as a glucose sensor that was applied for ex vivo glucose measurement (Figure 6b) [72]. The reversibility of 4-MPBA - glucose bonding allowed the continuous tracking of ambient glucose concentrations and made it suitable for biomedical applications [72]. In this context, AgNPs generate stronger SERS signals, offering better sensitivity compared to AuNPs [73; 74]. In several studies, it was found that the surface plasmons generated by silver nanoparticles are higher as compared to gold nanoparticles [73; 75]. [PERSON] et al. developed an SERS-based glucose-sensing substrate by coating polystyrene nanospheres with silver and mixing them with decanethiol (DT) and mercaptohexanol (MH). This system achieved a detection limit of 0.55 mM for glucose [48]. However, the larger linker molecules used in the system have a tendency to distance the glucose molecules from the surface's near-field enhancement, making it challenging to obtain consistent and precise measurements [72; 76]. Recently, [PERSON] et al. developed an SERS-based glucose nanosensor for in vitro measurements (Figure 7a). Their nanosensor probe reported a detection limit of 0.1 mM and was found to be linear in the range of 1-10 mM of glucose with excellent compatibility. The nanosensor was embedded into the tumor spheroid, in order to track the continuous metabolic activity in various tumor microenvironments (Figure 7b). The glucose in the intratumor microenvironment was measured for 5 days. The glucose gradient produced by the co-cultured colon cancer spheroid was found to be twice as high as that in the monocultured fibroblast spheroid (Figure 7c) [9]. This gives a very good insight into the tumor's metabolic activity, indicating how nutrient availability varies across the spheroid's diameter, with cells in the peripheral region receiving full nutrient coverage from the medium, while cells in the core region face significant nutrient shortages. This provides valuable insightinto the tumor's metabolic activity, indicating the relevance and importance of continuous glucose monitoring. Figure 6: **(a)** Schematic illustration demonstrating intradermal glucose detection using poly(methyl methacrylate) microneedle (PMMA MN) array. Reprinted with permission from [71]. **(b)** (i) Schematic illustration and SEM of Au-NP clusters stacked in a vertical ZnO NW pillars that later used as an SERS disk. **(ii)** Photo of SERS-disk mounted on C-shaped PDMS implant. Reprinted with permission from [72]. Figure 7: **(a)** The schematic illustration demonstrating the conjugation between SERS glucose nanoparticle sensor (gNPS) and glucose. The Raman intensity of SERS-gNPS was increased (at a wavelength of 1078 cm-1) post conjugation with glucose.