Ultra weak photon emission-a brief review

Abstract

Cells emit light at ultra-low intensities: photons which are produced as by-products of cellular metabolism, distinct from other light emission processes such as delayed luminescence, bioluminescence, and chemiluminescence. The phenomenon is known by a large range of names, including, but not limited to, biophotons, biological autoluminescence, metabolic photon emission and ultraweak photon emission (UPE), the latter of which shall be used for the purposes of this review. It is worth noting that the photons when produced are neither 'weak' nor specifically biological in characteristics. Research of UPE has a long yet tattered past, historically hamstrung by a lack of technology sensitive enough to detect it. Today, as technology progresses rapidly, it is becoming easier to detect and image these photons, as well as to describe their function. In this brief review we will examine the history of UPE research, their proposed mechanism, possible biological role, the detection of the phenomenon, and the potential medical applications.

Keywords: biological autoluminescence; biophoton; bystander effect; non-chemical signalling; radicals; ultraweak photon emission.

FIGURE 1

Mechanistic pathways of UPE from the mitochondria. The production of ROS, considered a byproduct of mitochondrial respiration, can lead to the production of singlet oxidation, or excited carbonyl species. When these excited intermediates relax to their ground states, they release this excess energy in the form of a photon. “Modified from Bókkon et al. (2010) (Figure 3)”.

FIGURE 2

EMCCDs need long exposures to image samples with a very low emission of photons. By binning pixel readout values a lower resolution but higher contrast image can be constructed. Image reproduced from Prasad and Pospíšil (2013) Figure 6B under CC-BY.

FIGURE 3

Comparison of possible readings from (left) digit photon readings per pixel and (right) camera background noise versus a single photon reading. A threshold applied will either exclude some single photon events or include some thermal events and therefore absolute quantitative readings are not possible from an EMCCD. Image reproduced with permission from (Zhang et al., 2009).

FIGURE 4

Typical Quantum Efficiency (QE) of a low dark count rate PMT [H11870-01Hamamatsu Photonics, Japan] Reproduced with permission from Ref (Mackenzie et al., 2024) Figure SI-4.