Understanding Upconversion Resonant Energy Transfer to pH-Sensitive Fluorescent Dyes: From Novel Core-Shell Luminescent Nanomaterials to Sensitive, Background-Free, and Broad-Range pH Sensing

Principal investigator: PhD Aleksandra Pilch-Wróbel

Globally, pH is one of the most commonly measured chemical parameters, and glass pH electrodes are widely used as a reference due to their reliability, precision, and broad measurement range. Despite their low cost and rapid response time, glass electrodes also have drawbacks, such as size, rigidity, and susceptibility to external fields, which limit their applicability in certain scenarios. Optical methods have emerged as effective alternatives for addressing the challenges of pH measurement, particularly at the nanoscale and for pH mapping.

The limitations of glass electrodes, including fragility and the need for storage solutions, have motivated researchers to explore alternative optical pH-sensing devices. FRET-based sensors, especially those utilizing optically active nanoparticles (UCNPs), show promise for measuring local changes in critical biological parameters. UCNPs, which emit higher-energy photons upon excitation with lower-energy photons, offer advantages such as photobleaching resistance and background-free detection. The characteristic multicolor emission of UCNPs enables ratiometric measurements, eliminating errors related to fluctuations in excitation intensity or nanoparticle concentration, making them a robust and accurate choice for diverse sensing applications.

However, although FRET plays a key role in the emission process, a potential challenge arises from photon reabsorption (PR) in optically active nanoparticles. PR occurs when a photon emitted by one UCNP is absorbed by a dye on another nanoparticle, leading to reduced emission and masking the true FRET response. This challenge arises from the random distribution of emissive ions throughout the UCNP volume, of which only a fraction near the surface participates in FRET. Many existing sensors overlook this complexity, relying on less-sensitive reabsorption processes.

The project proposes a promising solution based on core–shell architectures. By placing all donor ions in the shell, FRET energy transfer efficiency is increased while photon reabsorption is minimized. The primary aim of the project is to understand the energy-transfer mechanisms occurring in core–shell sensors. By studying different architectures (i.e., the distribution of ions within the shell) and dopant concentrations, a detailed analysis of potential energy-transfer pathways will be performed. The goal is to enhance the sensitivity of FRET-based pH sensors by reducing the simultaneous occurrence of PR. Positioning all donor ions within the FRET distance ensures complete energy transfer to the dye. The working hypothesis assumes that by employing an appropriate core–shell structure, potentially with all donor ions in the shell, FRET efficiency can be significantly increased, and PR minimized. Core–shell architectures not only enhance the efficiency of FRET-based sensors but also enable multifunctionality, revolutionizing nanoparticle design.

The second objective of the project is to develop multisensor nanoparticles for local measurement of pH and temperature or two distinct parameters simultaneously. The proposed approaches include:

1. Incorporating optically active ions as donors for different dyes, enabling simultaneous observation of two emissions.

2. Constructing a dual-function multisensor responsive to temperature and pH, using a core–shell structure with temperature-sensitive ions in the core and pH-sensitive ions in the shell.

This strategy will not only expand practical applications but also deepen understanding of the fundamental design principles for multisensor nanomaterials.