A promising strategy for the real-time monitoring and assessment of artificial fruit-ripening agents (AFRAs) is essential to protect human life and ecosystems (flora and fauna) from the harmful effects of these agents, as AFRAs are hazardous chemicals that can be found in fruits. Fruits serve as the main source of nutrition and are consumed by humans to a great extent to maintain health and fitness [[1], [2], [3]]. Owing to the rapidly increasing global population, there is an increasing demand for fruit; however, the production of fruits by natural means is limited. As a result, the needs of the rapidly growing population are not met, and fruits are artificially ripened to meet the high demand. Fruits are ripened using AFRAs, such as calcium carbide (CaC2), carbon monoxide, acetylene gas, ethephon, potassium sulfate, putrisein, potassium dihydrogen orthophosphate, photo porphyrinogen, and oxytocin [[4], [5], [6], [7], [8]]. Fruit ripening agents (FRAs) can be categorized into two types: natural and artificial. Natural ripening agents are naturally present on the skin of the fruit, causing the fruit to ripen instinctively for consumption. The natural ripening of fruits is a sequence of physiological, biochemical, and molecular activities that includes the synchronization of several metabolic processes, including the stimulation and inactivation of numerous genes, which generate variations in pigment, sweetness, tartness, texture, and fragrance volatiles [9]. The variation in pigments throughout the fruit ripening process results from exposure to colors due to chlorophyll breakdown, the production of different anthocyanins and their accumulation in vacuoles, and the presence of carotenoids [10]. Additionally, the synthesis of a dense combination of volatile compounds, including ocimene and myrcene, along with the absence of undesirable molecules like flavonoids, tannins, and additional associated compounds, enhances the flavor and fragrance of the fruit. Sugar levels increase due to enhanced gluconeogenesis (the metabolic route that synthesizes glucose), polysaccharide hydrolysis, reduced acidity, and the uptake of sugars and organic acids. Textural alterations in softened fruits occur because of enzyme-mediated modifications in the shape and cell wall components [11]. Through the above variations, the fruit becomes mature and ripe, with unique attributes: sweet, colorful, delicate, and appetizing. The only non-toxic and globally agreed-upon procedure is the use of ethylene, a natural ripening hormone; under regulated temperature and absolute humidity [12,13]. Notably, this process is quite slow compared to artificial ripening, which affects the supply chain of fruits in the market. Nonetheless, the fruits ripened using this process possess 100% nutritional value and remain fresh for extended periods (longer shelf life). Artificial ripening agents are produced in factories using harmful chemicals and are applied to the peel of preharvest fruits to induce ripening. This process is very fast and helps to maintain the flow of fruits in the market throughout the year. However, artificial ripening reduces the quality and nutritional value of fruits by blocking the biological pathway of natural fruit ripening [12,[14], [15], [16], [17]].

AFRAs are recognized as a global threat to human life and ecosystems owing to their carcinogenic potential. Most AFRAs are poisonous, and their ingestion can cause serious health complications, including dizziness, weakness, skin ulcers, heart disease, lung failure, memory loss, cerebral edema, depression, hematological changes, cancer, RNA, DNA, and quick-buck syndrome [18,19], and kidney failure (Fig. 1) [5,16,[20], [21], [22]]. AFRAs can also induce the growth of bacteria, viruses, and fungi, causing diarrhea, peptic ulcers, and other diseases [23]. All ripening agents contain high percentages of heavy metals that can cause significant morbidity and mortality. Almost every organ system is affected by heavy metal toxicity; however, the peripheral nervous system, central nervous system, hematopoietic, gastrointestinal, renal, and cardiovascular systems are the most frequently affected organs [22,24]. Depending on the specific heavy metal, age of a person, level and severity of toxicity, different organ systems may be affected. Children are more prone to accidental exposure to heavy metals and more vulnerable to hazardous consequences. For example, CaC2 transfers toxic trace elements, such as arsenic and phosphorus, to fruits during the artificial ripening process; these elements are then transferred to humans and animals, leading to uncommon health hazards [22]. AFRAs and their residues (heavy metals) are neurotoxic. Heavy metals exhibit an affinity towards nitrogen, sulfhydryl, and oxygen groups in proteins, causing variations in the activity of enzymes [22,[25], [26], [27]] such as acetylcholinesterase (AChE). AChE activity is reported to be deactivated or decreased by the heavy metals in AFRAs or their residues in the central nervous system in humans and insects. As a result, the neurotransmitter, acetylcholine, accumulates in the nerves and inhibits operations such as working activities and the functioning of muscles and important organs in the body, leading to severe symptoms or death [28].

Consumers should be more aware of the uncontrolled practice of artificial fruit ripening, especially its potentially harmful effects and the need for strict legislation to avoid health hazards [29]. Many countries have special legislative laws and regulations for AFRAs. Although many variations among the terms and conditions of the laws and legislation exist among different countries, these laws are purposely applied to monitor and ban the utilization of AFRAs [20,30,31]. Trade, purchase, and use of CaC2 in food are prohibited due to its hazardous properties [19,22,29]. A case study conducted in 2013 revealed the effects of accidental poisoning with CaC2. The diagnosis report revealed grade II mucosal ulcer excoriation on the area of the anterior chest wall, edema, and oral cavity due to contact with drooling saliva [29]. Notably, the fungicide Urbacid or Tuzet, which ripens apples, can cause various ailments, including skin rashes [32].

In the last few decades, protecting food, the environment, and human health has become a global need. To ensure protection, rapid and on-site portable sensing devices that can be used immediately before fruit export are needed to detect the presence of harmful substances [33]. The detection of AFRA in fruits at the lowest level recommended by the Environmental Protection Agency (EPA) is difficult to achieve using existing real-time detection technology. Therefore, effective portable on-site recognition systems for direct consumer use as 1st screening tools are necessary to control AFRA utilization [34]. The on-site AFRA detection technique is an approach for solving high health risk concerns and preventing farmers and vendors from incurring huge losses during fruit export. Conventional detection techniques are used to assess fruit quality and determine the food grade for human safety; however, their non-portability limits their use [[35], [36], [37]]. For decades, standard analytical methods (chromatographic techniques) and large instruments have been combined to detect the presence of AFRAs and have been proven to be benchmark techniques. These methods include gas chromatography (GC), high-performance liquid chromatography (HPLC), and other spectroscopic approaches, which have high sensitivity, reliability, and optimum efficiency [[38], [39], [40]]. Modern portable techniques, with several advancements, have been developed. Recent developments in sensing technique-based approaches have facilitated advanced technology development. The analysis of technological advancement has four generations; these advancements are categorized based on the innovation journey of AFRA detection techniques starting from the beginning to till date, including their advancements to cope up the challenges and increasing demand of portable kits (to boost their reliability, accuracy, reproducibility, stability, selectivity, sensitivity, rapidness, simple extraction and cleaning processes, cost-effectiveness, and reduced LODs), also challenges to overcome by characteristic modification and development of higher versions of advanced features to transform them into portable diagnostic kits so as to meet the necessity of end consumer for on-site detection, particularly in the field of AFRA detection: (i) 1st generation: conventional chromatographic techniques; (ii) 2nd generation: conventional spectroscopic techniques; (iii) 3rd generation: an advanced (rapid, robust, small instrument, not on-site) technique; and (iv) 4th generation: smart portable (rapid, robust, small kits, on-site, affordable) [[41], [42], [43]]. In the present scenario, “point-of-care sensors” (POCSs) have spread across various domains and sectors, such as healthcare, agriculture, and several other sectors. POCSs have solved the health safety goals (HSGs) of several nations [44]. The 4th generation portable techniques have attracted remarkable attention relative to previous generation detection techniques owing to their improved and new characteristics, such as affordability, sensitivity, specificity, stability, storability, user-friendliness, rapidity, robustness, equipment-free operation, and easy delivery to needy individuals (ASSSSURRED). Despite their advantage relative to previous techniques, the use of 4th generation portable techniques as well as studies to enable further improvements of these techniques must be promoted to achieve a more efficient and robust approach for AFRA detection. Few studies have focused on on-site detection approaches for ecosystem pollution monitoring and analysis of chemical residues, such as AFRAs, pesticides, drugs, and dyes. POC testing strategies and portable sensors, such as data processing techniques, optical techniques and paper-based techniques, have been developed for the early detection of AFRAs in fruits. However, no comprehensive analyses have been performed to elucidate the efficacy of on-site sensing techniques for AFRA residues, particularly in fruits. Furthermore, we provided a brief overview on the recent advances made in the wearable plant sensors for food science and technology.

In comparison with currently existing reviews on AFRAs detection techniques in fruits, none of the study is done on the smart portable detection techniques for AFRAs in fruits, highlighting all available techniques to date. Priyanka et al. addressed progress in deployed machine learning techniques in agriculture, application to varieties of banana fruit [45]. Anoopa et al. reviewed advancements in the use of image processing to create a fruit grading system based on machine vision, near-infrared spectroscopy, and hyperspectral imaging [46]. Jayaraman et al. encompassed numerous portable and lab-based techniques developed recently for the sensitive and particularly targeted detection of ethylene for fruit ripening applications [47].

To our knowledge, this review is the first to comprehensively discuss the modern state of on-site detection approaches and portable tools for the real-time detection of AFRA residues in fruits. Herein, chromatographic, data processing, optical imaging, and colorimetric detection approaches for sensing AFRA residues in fruits were evaluated. Thereafter, the analytical credibility of existing detection approaches was compared to confirm the efficiency of the paper-based real-time detection technique. Finally, new perspectives, the latest advancements, obstacles, and upcoming expectations were highlighted.