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Page 8 of 19 Hussain et al. Soft Sci. 2025, 5, 21 https://dx.doi.org/10.20517/ss.2025.02

increase in λ values as the urea concentration rises, with the shift plateauing when the C reaches
PBG
urea
50 mM. This linear trend highlights the sensor’s sensitivity to urea concentration changes, providing a
reliable optical response. Figure 1D shows the Δλ , with a maximum shift of 154 nm recorded,
PBG
demonstrating the significant modulation in photonic properties due to the swelling of the IPN upon urea
hydrolysis. Importantly, the typical concentration of urea in human sweat is approximately 22 mM for a
healthy individual . At this physiological concentration, the sensor maintains a green color, indicating
[52]
normal urea levels. However, as the concentration increases beyond normal physiological ranges, the sensor
exhibits a distinct color shift from green to yellow, and ultimately to red, signaling elevated urea levels. This
colorimetric change provides a straightforward visual cue for real-time urea monitoring. Supplementary
Figure 5C presents the UV-Vis spectra corresponding to the color shifts, further confirming the λ shifts
PBG
across different urea concentrations. The LOD for the CLCN-IPN urease film was calculated to be 0.273 mM,
with a linear range 0.7-50 mM, as derived from the data in Supplementary Figure 5D. The linear standard
curve in Supplementary Figure 5D was used to quantify the urea concentrations, demonstrating the sensor’s
high sensitivity and applicability for detecting even small variations in urea levels. The high sensitivity,
combined with the wide detection range, makes this biosensor a promising candidate for wearable
applications in health monitoring, particularly for tracking urea levels in sweat as an indicator of hydration
status or kidney function.

Lactate sensing by circular photonic CLCN-IPN biosensor film
The circular photonic CLCN-IPN biosensor film was fabricated using a 4% reactive CLC mixture, with the
IPN constructed from AA-co-DMAEMA. The IPN structure was specifically designed to facilitate dual
functionality: 20% of the AA units were utilized for the covalent immobilization of LOx enzymes, while the
remaining DMAEMA units were responsible for inducing a volumetric response to changes in pH. LOx
catalyzes the oxidation of L-lactate to pyruvate, a reaction that decreases the local pH. This is due to the pKa
values of lactate (3.86) and pyruvate (2.49), meaning the oxidation reaction produces a more acidic
environment, as illustrated in Figure 2A. For this reason, poly-DMAEMA was chosen as the hydrogel
matrix, as it is responsive to acidic pH, undergoing swelling and increasing in volume under such
conditions. The volumetric expansion of the hydrogel influences the helical pitch of the CLC, leading to a
detectable red shift in the λ . This enables the sensor to exhibit colorimetric changes across the visible
PBG
spectrum. Initially, the biosensor film displayed a blue color. To optimize lactate sensing, different
concentrations of LOx (C ) were immobilized on the CLCN-IPN film and tested using a 50 mM lactate
LOx
(C Lactate ) aqueous solution. Figure 2B shows that the λ values of the CLCN-IPN film increased linearly
Lox
PBG
with increasing C , reaching equilibrium at 8 µM. The inset photographs demonstrate a visible color shift
LOx
from blue to yellow as the C concentration increased. Supplementary Figure 6A presents the
LOx
corresponding UV-Vis spectra of CLCN-IPN films with varying C concentrations, displaying a
LOx
Lox
continuous red shift in the λ until C exceeds 8 µM. Supplementary Figure 6B illustrates the Δλ values,
LOx
PBG
PBG
with a maximum shift of 136 nm observed for films immobilized with 8 µM or higher concentrations of
C . We selected 8 µM of LOx as the optimal concentration for all experiments unless otherwise specified.
LOx
The performance of the CLCN-IPN sensor was evaluated using aqueous solutions of lactate (C Lactate ) at
Lox
varying concentrations. Figure 2C shows a redshift in λ values as the concentration of C Lactate increases.
PBG
Specifically, the λ shifted from 435 to 579 nm, reaching equilibrium at a C Lactate concentration of 50 mM.
PBG
The inset photographs display a distinct color change of the film, transitioning from blue to brown. The
physiological concentration of lactate in human sweat is typically around 25 mM, which corresponds to
moderate physical exertion. At this normal physiological level, the sensor exhibits a green color. However,
as lactate levels increase, such as during intense exercise or pathological conditions the sensor transitions to
yellow and eventually brown, indicating elevated or potentially toxic levels of lactate. This colorimetric shift
provides an intuitive, real-time indication of lactate levels in sweat. Lactate is a byproduct of anaerobic