**1. Introduction**

Reactive Oxygen Species (ROS) is broadly used for those which contain oxygen radical such as superoxide anions (O2 .−), hydroxyl radicals (HO. ), hydrogen peroxide (H2O2), nitric oxide (NO), peroxynitrite (ONOO− ), hypochlorous acid (HOCl) and singlet oxygen (1 O2)(SO). These are usually metabolites of molecular oxygen formed due to normal or abnormal redox function in human body [1]. Numerous studies suggested that ROS can acts as toxins which can reduce the life span by causing ageing [2], synovial fluid degradation [3], neurodegenerative disease [4] etc. Singlet oxygen is known to be a highly reactive oxygen species that can oxidise a broad range of

#### **Figure 1.**

*The various photophysical singlet oxygen detection techniques based on absorbance or luminescence probes have been described pictorially. From top left clockwise (a) fluorescence detection probe (1,3-diarylisobenzofuran, adapted from [13]), (b) UV–Vis absorption based spectrophotometric probe (bis-9,10-anthracene-(4-trimethylphenylammonium) dichloride, adapted from [14]), (c) chemiluminescence probe (SOCL-phenol-dioxetane species, adapted from [15]) and (d) rare earth photoluminescence probe, ([4*′*-(9-anthryl)-2,2*′*:6*′*,2*″*-terpyridine-6,6*″*-diyl]bis (methylenenitrilo) tetrakis (acetate)-Eu3+ (ATTA-Eu3+) (adapted from [16]).*

biomolecules including lipids [5], nucleic acids [6, 7] and proteins [8]. The oxidation chemistry of SO provide key mechanism in the regulation of intracellular signalling pathways [9, 10] and eventually connected to human pathophysiology. As SO is a very short lived species (τ1/2 = 10−6 to 10−5 s) in the aqueous milieu) [11] studies towards the molecular mechanism of <sup>1</sup> O2 in vivo has been significantly retarded. Direct monitoring of SO phosphorescence also lacks practical utility due to very low photoluminescence quantum yields (PLQY≈10−6) [12]. Therefore an enormous demand for SO probe with high sensitivity and large dynamic range is required. In this article various photophysical techniques and related probes usually employed in detecting SO has been delineated in separate paragraph (**Figure 1**).
