Progesterone (P4) is a kind of natural steroid, which is mostly secreted by corpus luteum[1, 2]. It plays an important role in reproductive tissue, menstrual cycle regulation, fetal growth and early pregnancy[2]. Nowadays, many clinical studies use the amount of P4 to measure the hormone level in the body[3]. However, when the amount of P4 is too high, it will cause weight gain, breast discomfort[4], mental depression, acne and other side effect[5]. Epidemiological studies have confirmed that the imbalance of progesterone levels has an impact on the incidence of breast and cervical cancer[6]. As many scholars have studied progesterone and other hormones, they have found that not only the hormones in the body, but also the female hormones in food can also have an impact on human health. Long-term consumption of foods containing progesterone, even low-concentration foods, will interfere with the normal function of human body secretion and immune system[7]. Especially milk, milk contains a considerable amount of estrogen, and progesterone is concentrated in fat[8]. When the amount of P4 in milk is too high, it will lead to breast cancer and other diseases[9]. Therefore, the detection of P4 content is of great practical significance. At present, the main detection methods of P4 are high performance liquid chromatography[10], fluorescence spectrophotometry[11], electrochemical method and so on[12, 13]. Most of these methods need expensive instruments, complex pretreatment process, cumbersome analysis process and time-consuming, so they are not suitable for large-scale field detection. However, because of the advantages of simple operation, high sensitivity and low cost, fluorescence spectrophotometry has caught the attention of researchers.
On the basis of fluorescence spectrophotometry, fluorescent sensors based on CDs have attracted more and more attention due to their fast and simple operation, excellent fluorescence performance, less harsh storage conditions and low toxicity. Carbon dots (CDs) are carbon nanoparticles with particle size less than 10 nm, which can stably emit light[14]. At present, CDs have been used in the fields of biological imaging[15], biosensor[16], ion molecule detection[17], information anti-counterfeiting storage and so on[18]. Trapiella et al. prepared CdSe/ZnS quantum dots as a fluorescent probe to detect progesterone in milk[19]. Based on the toxicity of semiconductor quantum dots, and the relatively complex synthesis method. Cao et al. developed an "on-off" fluorescence sensor based on CDs to detect progesterone, which reflects the advantages of carbon dots[20], but the sensitivity and selectivity of this method are not high enough. Therefore, how to design a high-sensitivity and high-selectivity fluorescence sensor based on carbon dots to detect progesterone needs further exploration.
According to the literature, aptamers have the advantages of high specificity, high affinity, wide target range, easy modification, short preparation time, high stability and low toxicity[21, 22]. Therefore, we tried to combine carbon dots with aptamers to improve the sensitivity and selectivity of the sensor. Aptamer is a small oligonucleotide sequence or short polypeptide that can specifically recognize the target obtained by systematic evolution of ligands by exponential enrichment (SELEX) technology[23–25]. As a new type of biosensor identification element, it can be applied to the detection of food[26], drugs[27], viruses and other aspects[28], and also have a broad application in the field of biological imaging[29]. This work combines carbon dots with aptamers, that is, combines the excellent fluorescence performance and low toxicity of carbon dots with the advantages of aptamers for specific detection of the detection substance, and constructs a simple and convenient fluorescence aptamer sensor.
Therefore, this paper constructed a fluorescent aptamer sensor based on carbon dots and graphene oxide to detect P4. Among them, graphene oxide acts as a quencher to form an excellent donor-acceptor pair in the fluorescence resonance energy transfer (FRET) process with carbon dots. Energy transfers from the surface of the carbon dots to graphene oxide, resulting in fluorescence quenching of the carbon dots. When P4 is present, it specifically binds to the aptamer on the surface of the carbon dots, and the carbon dots fall off from the surface of graphene oxide, causing the fluorescence of the carbon dots to recover. Experiments show that the sensor has many advantages such as easy operation, high selectivity, high sensitivity, wide detection range, and low detection limit.