NQS-Doped PDMS Solid Sensor: From Water Matrix to Urine Enzymatic Application

Abstract

The development of in situ analytical devices has gained outstanding scientific interest. A solid sensing membrane composed of 1,2-naphthoquinone-4-sulfonate (NQS) derivatizing reagent embedded into a polymeric polydimethylsiloxane (PDMS) composite was proposed for in situ ammonium (NH₄⁺) and urea (NH₂CONH₂) analysis in water and urine samples, respectively. Satisfactory strategies were also applied for urease-catalyzed hydrolysis of urea, either in solution or glass-supported urease immobilization. Using diffuse reflectance measurements combined with digital image processing of color intensity (RGB coordinates), qualitative and quantitative analyte detection was assessed after the colorimetric reaction took place inside the sensing membrane. A suitable linear relationship was found between the sensor response and analyte concentration, and the results were validated by a thymol-PDMS-based sensor based on the Berthelot reaction. The suggested sensing device offers advantages such as rapidity, versatility, portability, and employment of non-toxic reagents that facilitate in situ analysis in an energy-efficient manner.

Keywords: NQS-PDMS sensor; ammonium; glass support; in-situ analysis; optical sensor; urea; urea hydrolysis; urease; urine; water.

Figure 1

Steps of the sol-gel process that takes place for the sensor preparation.

Figure 2

NH₄⁺ (50 mg L⁻¹) standard solutions: (a) Variation of the colorimetric signal as a function of the NQS content and exposure time at room temperature. (b) Variation of the analytical signal as a function of the temperature for 5 and 10 min reaction times. (c) Variation of sensor response depending on the SiO₂ NP composition tested. (d) Analytical response as a function of the pH and exposure time; inset: images of the sensing device in blank solution (yellow) and NH₄⁺ standard solution 1.5 mg L⁻¹ (brownish) after 5 h exposure time at room temperature. (e,f) SEM images of the synthesized NQS-PDMS sensing membrane, scale bar: 20 µm (e) and 30 µm (f).

Figure 3

(a) Difference in UV-Vis spectra between standards of 2.5 mg L⁻¹ NH₄⁺ (black line), 9 mg L⁻¹ NH₄⁺ coming from hydrolyzed urea (orange line), and 1600 mg L⁻¹ urea (blue line) and the blank standard. (b) Top: Optical microscopy images of the PDMS membrane before (left) and after (right) being exposed to ammonium/urea in solution, scale bar: 20 µm. Bottom: Sensor photos referring to the evolution of the sensor color as a function of ammonium or urea concentration in solution.

Figure 4

Schematic diagram of covalent urease immobilization on a borosilicate glass surface involving surface functionalization with APTMS.

Figure 5

Calibration curves for hydrolyzed urea standards (blue) and spiked urine samples (n = 2, orange and grey) for urea catalysis by means of (a) urease enzyme in solution and (b) glass-supported urease immobilization.