# A host–guest approach to ratiometric pH sensing using upconversion nanoparticles

## TL;DR

Researchers developed a reusable and selective pH nanosensor using upconversion nanoparticles and a pH-sensitive dye for ratiometric sensing in a specific pH range.

## Contribution

A novel ratiometric pH nanosensor using host–guest interactions and upconversion nanoparticles for stable and selective sensing.

## Key findings

- The sensor shows a ratiometric response (blue/red) over a pH range of 8.0–11.0 with high reproducibility.
- The nanoplatform demonstrates excellent reusability and selectivity in the presence of interferents.
- The red emission (650 nm) remains stable and is used as an internal reference for ratiometric sensing.

## Abstract

Herein, we present a pH-sensitive nanoplatform based on Tm3+/Yb3+ co-doped upconversion nanoparticles (UCNPs) functionalized with β-cyclodextrin (β-CD) and a pH-responsive nitrobenzoxadiazole dye modified with adamantane (NBD-Ad). The host–guest interaction between β-CD on the UCNP surface and Ad on NBD-Ad enables a stable, water-dispersible ratiometric pH sensor. Upon 980 nm excitation, the UCNPs emit in the UV and blue regions, which overlap with the pH-dependent absorption of NBD-Ad. This overlap induces selective quenching of UV and blue emissions via inner filter effect (IFE). The red emission (650 nm) remains stable and is used as an internal reference for ratiometric sensing. The sensor shows a ratiometric response (blue/red) over a pH range of 8.0–11.0 with high reproducibility. The nanoplatform demonstrates excellent reusability and selectivity even in the presence of interferents.

A new pH nanosensor based on β-NaYF4:Yb3+/Tm3+ upconversion nanoparticles functionalized with cyclodextrin and a pH-sensitive adamantane-modified nitrobenzoxadiazole dye (NBD-Ad).