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(a) Baseline-corrected SERS spectra of urea sensing hydrogel immersed in buffer (10 mM Tris-maleate buffer originally at pH 7 with 10 mM CaCl2) containing urea concentrations 0, 0.1, 1, 10 mM, 60 min after immersion. (b) Time dependent ratio response for urea sensing hydrogel for 0, 0.1, 1, and 10 mM urea. (c) Urea calibration curve corresponding to SERS spectra in (a).
