Publications

@conference{Vu2026,

title = {Electrohydrodynamic Fabrication of Silica Microtips for SERS-Based Microscale pH Sensing},

author = {M. C. Vu and V. T. Dau and P. Papp and R. V. Goreham and A. Vinu and A. J. Fleming and Y. K. Yong},

year  = {2026},

date = {2026-02-03},

urldate = {2026-02-03},

booktitle = {International Conference on Nanoscience and Nanotechnology},

abstract = {Background and aims



Accurate detection of microscale pH variations is essential for understanding localized chemical and biological processes in heterogeneous systems such as tissues, hydrogels, and pharmaceutical formulations1. Conventional pH electrodes lack the spatial resolution required for these measurements2. This work aims to develop a facile, low-cost electrohydrodynamic (EHD)3 fabrication route for producing ultralong silica microtips, which are subsequently functionalized into surface-enhanced Raman scattering (SERS)-active probes for localized pH sensing.



Methods 

Silica sol–gel precursors were processed using a direct-current EHD jetting method to form conical microtips with apex diameters below 5 µm. Tip geometry was tuned by controlling precursor viscosity, needle gauge, and applied voltage. The microtips were coated with gold nanoparticles before functionalization with 4-mercaptopyridine (4-MPy), a pH-responsive Raman reporter. The SERS response was calibrated in buffer solutions (pH 3.0–10.0) using the intensity ratio of the 1006 cm-1 and 1097 cm-1 bands4. 



Results 

Optimized conditions (94 cP viscosity, 3.0 kV) produced uniform ultralong microtips of up to 1 mm in length with apex diameters near 3 µm. The EHD process enabled reproducible formation of stable Taylor cones and controllable jet elongation. The resulting SERS microtips exhibited a linear ratiometric correlation (R2 = 0.993) between I1006/I1097 and pH, demonstrating high sensitivity and reproducibility. When applied to semi-solid hydrogels, the microtips resolved local pH variations with low deviations, outperforming bulk electrode measurements.



Conclusion 

This study demonstrates that EHD-fabricated silica microtips can be reliably converted into SERS-active pH sensors offering high spatial resolution, chemical specificity, and mechanical robustness. The technique provides a scalable platform for microscale chemical sensing with potential applications in pharmaceutical quality control, biomedical diagnostics, and nanomedicine.},

keywords = {Electrohydrodynamics, Sensors},

pubstate = {published},

tppubtype = {conference}

}

@conference{Hoang2025,

title = {Free-templated Microfabrication of Hydrogel Microneedles Using Electrohydrodynamic Technique for Transdermal Drug Delivery},

author = {T. A. Hoang and H. D. Vu and D. Ngo and N. L. Mai and C. Doan and D. K. Tran and Y. Xiao and B. Rehm and Y. K. Yong and V. T. Dau},

year  = {2025},

date = {2025-11-03},

urldate = {2025-11-03},

booktitle = {The 29th International Conference on Miniaturized Systems for Chemistry and Life Sciences -

Micro-Total Analysis Systems (uTas)},

address = {Adelaide, 2 - 6 November, 2025},

abstract = {This study presents a template-free electrostreching microfabrication method for hydrogel microneedles, targeting applications in wound healing and drug delivery. Precursor solutions of gelatin methacryloyl (GelMa) and hyaluronic acid methacryloyl (HAMA) were subjected to a high-voltage electric field, forming a hydrogel microneedle structure by undergoing UV-crosslinking and solvent evaporation. The fabricated microneedles demonstrated optimal geometries, i.e., height-to-base ratios and the ability to be grown in parallel. In vitro cytotoxicity test confirmed the biocompatibility of the microneedles, as cells were able to adhere and proliferate on their surfaces.



Traditional transdermal drug delivery approaches, including hypodermic needles, topical creams, and transdermal patches, have significant drawbacks such as slow drug absorption and the painful and discomfort injection process [1], [2], [3]. Microneedles, which are arrays of micron-scale needles arranged in a small patch, offer an effective alternative solution by combining a non-invasive method and fast onset action to enhance patient compliance and potential for self-administration. Although there are various types of microneedles, most are fabricated using template-based methods such as moulding, lithography, or top-down micromachining, which require complicated processes in cleanroom facilities and limit design flexibility. To address these limitations, we report a template-free fabrication approach using electrohydrodynamic principles to produce microneedles directly on multiple substrate materials. Building upon prior work introduced in Small Methods [4], which focused on a proof-of-concept demonstration, this study extends the application to biocompatible hydrogels and evaluates the system’s scalability and safety for transdermal drug delivery. The method enables parallel microneedle fabrication and supports cell viability, highlighting its potential for practical therapeutic applications.



An overview of microneedle fabrication is illustrated in Figure 1. A hydrogel solution containing GelMa, HAMA, and the photo-initiator LAP (Lithium phenyl-2,4,6-trimethylbenzoylphosphinate) was prepared and dispensed as a droplet onto a substrate positioned between the two high-voltage electrodes. Under the applied pulsating electric field  , the droplet was elongated and stretched into a conical shape (Taylor-cone shape). The droplet was then exposed to UV light to initiate hydrogel photo-polymerisation, which solidified the droplet’s outer layer. By the subsequent air-dry process, the solvent completely evaporates, resulting in a solid microneedle. This method enabled parallel microneedle formation, as shown in Figure 2, where five microneedles were simultaneously grown with consistent form and geometry. Figure 3 demonstrates the in-situ adaptability of the method, showcasing its ability to grow microneedles directly on various commercially available adhesive patches. GHMa-1, -2, and -3 samples exhibited desirable geometries, with height-to-base ratios from 1.90 to 3.23 (Table 1). Biocompatibility of the microneedle was validated by culturing cells (RAW 264.7) in direct contact with the microneedle, where cell adhesion and proliferation were observed (Figure 4). These findings underscore the potential of this technique for scalable and flexible fabrication of biocompatible hydrogel transdermal drug delivery platforms.},

keywords = {Electrohydrodynamics, Microneedles, Pulsed voltage},

pubstate = {published},

tppubtype = {conference}

}