Recognizing and diagnosing the disease at an early stage is essential in therapeutic practice. In particular, scary diseases such as cancer can be effectively dealt with if detected early [1]. According to reports, the survival rate of gastric cancer patients increased from 33.3% to 92.3% when abnormalities were found in the early stages of the disease [2]. Similar studies have also been reported on patients with breast cancer, with early detection reducing mortality by 17–28% [3]. Biomedical imaging is often the most powerful and efficient tool when diagnosing disease and monitoring treatment progress.

Nanomaterials have recently become one of the most lucrative research topics for biomedical imaging researchers. Its unique passive, active, and physical targeting capabilities have sparked efforts to advance biological detection and imaging. The nanoparticles act as efficient contrast agents and exhibit efficient permeability and retention in tumors to differentiate between normal and tumor tissue [4]. Among all other characteristics of nanoparticles, nanoparticle size plays an important role in imaging applications and substantially affects properties such as biodistribution, cellular uptake, circulatory half-life, and cellular penetration [5]. Besides bioimaging, several studies have reported using nanoparticles for therapeutic applications such as drug and gene delivery [6], [7] and photodynamic therapy [8]. Removing the smaller-sized nanoparticles from the body after use is also much easier. Since the average size of renal filtration pores is larger than 10 nm, the renal excretory system efficiently eliminates nanoparticles smaller than 10 nm [9], [10]. These nanoparticles, typically smaller than 10 nm, are known as quantum dots (QDs).

The researchers defined quantum dots as "small crystals containing a variable number of electrons that occupy well-defined discrete quantum states and have electronic properties intermediate between those of bulk and discrete elementary particles"[11], [12]. Technically, the term 'quantum' refers to a discrete unit of physical property. The properties of quantum dots are determined by quantum mechanics due to the motion of electrons confined in three dimensions [12]. Due to their unique optoelectronic properties, semiconducting quantum dots such as zinc oxide (ZnO), zinc sulfide (ZnS), cadmium selenide (CdSe), cadmium telluride (CdTe), and many others [13] have found widespread use in non-contact applications such as semiconductors [14], solar cells [15], thin-film transistors [16], environmental sensing [17], [18], [19], and so on. The emission wavelength of QDs is size-dependent and can be tuned through scaling. Biomedical applications such as cellular imaging [20], [21] have used the photoluminescent capabilities of the QDs over the past decade [22], [23]. However, research on low-atomic-weight non-metallic elements such as carbon, phosphorus, sulfur, and silicon is currently being conducted as an alternative to heavy metal-semiconductor quantum dots. The advent of these environmentally friendly and low-toxic quantum dots has made them a desirable option for use in applications that come in direct contact with living things and humans [24]. Sulfur quantum dots have become increasingly popular because they are mild, sustainable, extremely fluorescent, and inexpensive [25].

Although several reports have recently been published on bioimaging applications of sulfur quantum dots, other essential properties of SQDs make them suitable for other biomedical applications, too (Fig. 1). This review article focuses on the recently reported biomedical applications of SQDs. It also provides insight into the synthesis process and a perspective on other beneficial properties of SQDs, such as antibacterial and antioxidant activity, which can be explored and applied in other biomedical fields.