We present a method of making low-loss split-ring resonators (SRRs) for microfluidic sensing at microwave frequencies using silver-coated copper wire. We show that a simple geometric modification and the use of square cross-section wire give greater electric field confinement in the capacitive region of the resonant sensor. We use a combination of theoretical analysis, finite-element simulations, and empirical measurements to demonstrate the subsequent increases in the sensitivity of these SRRs for complex permittivity measurements of some common solvents.