# Radiation Brightening from Virus-like Particles

## TL;DR

This study demonstrates that virus-like particles with densely packed chromophores exhibit radiation brightening under pulsed excitation, overcoming typical concentration quenching and enabling potential new biophotonic imaging applications.

## Contribution

It reveals a novel phenomenon of radiation brightening in virus-like particles with high chromophore density, driven by collective relaxation mechanisms under pulsed excitation.

## Key findings

- Brightening occurs near maximum surface site occupancy.
- Steady-state excitation shows conventional quenching.
- Heterogeneity suppresses radiation brightening.

## Abstract

Concentration quenching is a well-known challenge in many fluorescence imaging applications. Here we show that the optical emission from hundreds of chromophores confined onto the surface of a virus particle 28 nm diameter can be recovered under pulsed irradiation. We have found that, as one increases the number of chromophores tightly-bound to the virus surface, fluorescence quenching ensues at first, but when the number of chromophores per particle is nearing the maximum number of surface sites allowable, a sudden brightening of the emitted light and a shortening of the excited state lifetime are observed. This radiation brightening occurs only under short pulse excitation; steady-state excitation is characterized by conventional concentration quenching for any number of chromophores per particle. The observed suppression of fluorescence quenching is consistent with efficient, collective relaxation at room temperature. Interestingly, radiation brightening disappears when the emitters' spatial and/or dynamic heterogeneity is increased, suggesting that the template structural properties may play a role and opening a way towards novel, virus-enabled imaging vectors that have qualitatively different optical properties than state-of-the-art biophotonic agents.