# Biocompatible and Flexible Cellulose Film for the Reversible Colourimetric Monitoring of pH and Mg (II)

## TL;DR

A flexible and biocompatible cellulose film was developed to detect pH and magnesium ions with high accuracy and reversibility, suitable for wearable and real-time monitoring.

## Contribution

A novel cellulose-based film with covalently immobilized Hyphan I chromophore for reversible and selective sensing of pH and Mg2+.

## Key findings

- The CFH film shows fast and fully reversible colorimetric response to pH and Mg2+ over physiologically relevant ranges.
- The film has high selectivity for Mg2+ over Ca2+ and common physiological ions.
- The material is transparent, flexible, and biocompatible, suitable for integration into wearable and microfluidic systems.

## Abstract

What are the main findings?
Fast, reversible colourimetric sensing of pH and Mg2+ with high selectivity over Ca2+ and common physiological ions.Transparent, flexible, and biocompatible cellulose-based thin film material suitable for continuous real-time and wearable optical sensing.

Fast, reversible colourimetric sensing of pH and Mg2+ with high selectivity over Ca2+ and common physiological ions.

Transparent, flexible, and biocompatible cellulose-based thin film material suitable for continuous real-time and wearable optical sensing.

What are the implications of the main findings?
Covalent immobilisation of Hyphan I chromophore on cellulose via vinylsulfonyl chemistry.Stable, non-leaching sensor platform enabling scalable and cost-effective fabrication.

Covalent immobilisation of Hyphan I chromophore on cellulose via vinylsulfonyl chemistry.

Stable, non-leaching sensor platform enabling scalable and cost-effective fabrication.

Novel colourimetric sensors are readily devised by combining multifunctional (nano)materials with miniature optoelectronic components. The demand to detect and monitor metal ions has resulted in the invention of new colourimetric sensing schemes, especially for use at the Point-of-Need (PoN). Nonetheless, the design of fully reversible optical materials for continuous real-time ion monitoring remains a bottleneck in the practical realisation of sensors. Magnesium ion is vital to physiological and environmental processes, but monitoring can be challenging, particularly in the presence of Ca2+ as a cross-sensitive interferent in real samples. In this work, a chromophore molecule Hyphan I (1-(2-hydroxy-5-ß-hydroxyethylsulfonyl-phenyl-azo)-2-naphthol) has been grafted onto a cellulose matrix with a simple one-pot vinylsulfonyl process, to form a transparent, biocompatible and highly flexible thin-film colourimetric magnesium ion sensing material (Cellulose Film with Hyphan-CFH). The CFH film has a pH response time of <60 s over the pH range 4 to 9, with a pKa1 = 5.8. The LOD and LOQ for Mg2+ at pH 8 are 0.089 mM and 0.318 mM, respectively, with an RSD = 0.93%. The CFH film exhibits negligible interference from alkaline and alkaline earth metals, but irreversibly binds certain transition metals (Fe3+, Cu2+ and Zn2+). The CFH material has a fast and fully reversible colourimetric response to pH and Mg2+ over physiologically relevant ranges without interference by Ca2+, demonstrating good potential for integration into microfluidic systems and wearable sensors for biofluid monitoring.