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Chemistry Africa (2020) 3:181–188 https://doi.org/10.1007/s42250-019-00105-7

ORIGINAL ARTICLE

Simple Colorimetric and Fluorometric Assay Based on 2,3‑Naphthalenedialdehyde for Melatonin in Human Saliva

Bicheng Liu1 · Yashi You1 · Daiqin Lin2 · Zongbao Chen3 · Ping Qiu1

Received: 18 August 2019 / Accepted: 4 November 2019 / Published online: 13 November 2019 © The Tunisian Chemical Society and Springer Nature Switzerland AG 2019

AbstractIn this paper, 2,3-naphthalenedialdehyde (2,3-Nda) was selected as color reagent with colorimetric and fluorometric assay for detecting melatonin in human saliva. 2,3-naphthalenedialdehyde reacted with melatonin under the catalysis of hydro-chloric acid and Fe3+, which the color change was observed sensitively from colorless to yellow with the naked eye. On the other hand, the product of the reaction had a large conjugate structure with strong fluorescence for the fluorometric analysis. Under optimized experimental conditions, a concentration range of melatonin was 2.5–37.5 µM (R2 = 0.998) by colorimetry with the limit of detection for 1.288 µM (S/N = 3), and 0.01–0.1 µM (R2 = 0.997) by fluorometry with the limit of detection for 0.004 µM (S/N = 3). The two assays were successfully applied to the determination of melatonin in human saliva, which average recoveries were 97.80% and 95.96%, respectively, which has the potential in fast clinical examination of melatonin in human saliva.

Keywords Colorimetry · Fluorometry · 2,3-Naphthalenedialdehyde · Melatonin

1 Introduction

Melatonin (C13N2H16O2, MT) is a neuroregulatory hormone secreted by the pineal gland [1, 2]. It belongs to indoles het-erocyclic compound, and its chemical name is N‐acetyl‐5‐methoxytryptamine, also known as pineal hormone or mela-tonin [3]. MT is a kind of body hormone that induces natural sleep, and it can overcome sleep disorder and improve sleep quality by regulating people’s natural sleep [4–6]. And MT can directly act on the gonads, reduce the content of andro-gen, estrogen and progesterone [7, 8]. In addition, MT has strong neuroendocrine immune regulatory activity, which may become a new antiviral treatment method and approach [9–13]. And it has free radical scavenging antioxidant abil-ity, which slow down the aging of human organs [14–16]. Most importantly, MT has a good effect of inhibiting cancer

[17–19]. It was reported that people with abnormally low MT levels are more likely to have prostate cancer and breast cancer [20, 21]. What’s more, MT has an ability to reduce the toxicity during chemotherapy, which protects the health of human normal cells and organs, to a certain extent [22, 23]. The level of melatonin in the body is very low, and it exists in saliva at 2.4-12.4 pg mL−1 level [24]. The secre-tion of melatonin at the night is 5–10 times more than that in the day, which reaches the peak from 24:00 to 5:00 am. The level of MT at night directly affects the quality of sleep [25–27]. So many assays of detecting melatonin have been developed continuously.

Many methods for the detection of melatonin in body fluids have been reported, such as high performance liquid chromatography (HPLC) coupled to fluorescence detector [28], gas chromatography mass spectrometry (GC–MS) [29, 30], HPLC–LC/MS [31, 32], capillary electrophoresis [33], radioimmunoassay (RIA) [34], enzyme-linked immuno-sorbent assay (ELISA) [35, 36], colorimetric sensor arrays [37–39]. Among them, ELISA kit for detection of melatonin is commercially available and the LOD was 0.5 pg mL−1 [40]. However, there are few reports about fluorescence of determination melatonin in human body.

MT, as mentioned above, belongs to the Indoles het-erocyclic compounds with a free C-2 position. It has

* Ping Qiu pingqiu@ncu.edu.cn

1 Department of Chemistry, Nanchang University, Nanchang 330031, China

2 Jiangxi Province Product Quality Supervision Testing Institute, Nanchang 330047, China

3 School of Chemistry and Environmental Science, Shangrao Normal University, Shangrao 334001, China

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been reported that the electron density of this position is strong. In the previous work [37], sensitive and accurate colorimetric and fluorometric methods were presented for the determination of melatonin in tablets and serum. The authors utilized the reactions of p-dimethylaminoben-zaldehyde in hydrochloric acid (van Urk reagent)-ferric chloride in sulphuric acid for colorimetry and the reac-tion of melatonin with o-phthalaldehyde in acid medium which yields high fluorescent condensation product for fluorometry.

Aromatic compounds are usually view as fluorescent probes because of its conjugated structures. Aldehyde-decorated fluorescence probe is an important and excel-lent probe because of the activity of the aldehyde group, which can specially identify certain groups or ions [37, 41]. 2,3-Naphthalenedialdehyde (2,3-Nda) with the struc-ture of naphthalene, has two aldehyde group as reactive positions, which are easy to be attacked by a nucleophile. Matsufuji’s group [42] developed a fluorometric method for angiotensins (ANG I, II, III) in human plasma based on 2,3- Nda with highly reproducible and precision, which excitation and emission wavelengths were set at 420 nm and 490 nm, respectively. There are also reports in the literature which 2,3-Nda was applied to detect cyanide based on the reaction of cyanide, 2,3-Nda and taurine. And the proposed method was applied success-fully to blood analysis [43, 44]. It was also reported that several NDA derivatives was designed and synthesized, which had been successfully used to detect glutathione in HeLa cells [45].

MT can be detected in the blood, urine and saliva [46, 47]. There is no doubt that saliva sampling is the better choice because it is non-invasive, non-painful and easy to obtain [48]. Salivary fluid is composed of water (more than 95%), and of various electrolytes, hormones, enzymes, immunoglobulins, cytokines, etc. [49]. There are also glucose and nitrogenous products, such as urea and ammonia [50]. Most of the mucins may be denatured by freezing and thawing the sample; the denatured mucins also tend to adsorb much of the debris from the sample and the freezing eliminates most of the froth associated with a fresh saliva sample [51]. When saliva samples treated with anhydrous ethanol, these proteins denature. After centrifu-gation, the denatured protein is removed as a precipitate. The components become single and there are basically no interfering factors, supernatant becomes relatively simple.

In this paper, simple colorimetric and fluorometric assay was developed for melatonin in human saliva using 2,3-Nda as a chromogenic reagent. The reagent reacted with MT under acidic conditions with FeCl3 as the cat-alyst, which the color of solution changed from color-less into yellow. On the other hand, the product has a large conjugated system with strong fluorescence for the

fluorometric analysis. Finally, the assay was applied to the determination of melatonin in saliva with the average recovery of colorimetry and fluorometry for 97.80% and 95.96%, respectively. The methods are simple and low cost for detection of MT.

2 Materials and Methods

2.1 Reagents and Instruments

Melatonin (98%) was purchased from Solarbio® (Bei-jing, China). Stock standard solution of melatonin (0.2000 mmol L−1) was prepared in ethanol and stored refrigerated at 4 °C in brown glass flasks. 2,3-Naphtha-lenedialdehyde was purchased from Tokyo Chemical Industry Co. Ltd (Japan). H2SO4, HCl and absolute ethyl alcohol were obtained from XiLong Scientific (Guang-dong, China). All chemicals are analytical grade and do not require further treatment. Ultrapure water (18.25 MΩ, Millipore, USA) are used throughout the whole experi-ment. 2,3-Nda (2.0000 mmol L−1) was prepared in mix-ture of concentrated HCl (s.g.1.19) and ethanol (HCl: ethanol = 1:1), and FeCl3 (0.5 mmol L−1) was prepared in a mixture of sulfuric acid and H2O (H2SO4:H2O = 3:5). Above stock standard solution was advised to store refrig-erated at 4 °C in brown glass flasks, and diluted into dif-ferent concentrations that we need.

CR-I was prepared for colorimetry, which was made up of 2,3-Nda (2.00 mmol L−1) and FeCl3 (0.05 mmol L−1) with the ratio of 4:1. CR-II was prepared for fluorometry similarly, which was made up of 2,3-Nda (1.00 mmol L−1) and FeCl3 (8.0 × 10−5 mmol L−1) with the ratio of 1:4.

UV–vis absorption spectra were recorded using UV-2450 Shi-madzu Vis-spectrometer (Japan). The fluo-rescence spectra were performed on a F-4600 fluorescence spectrometer (Hitachi, Japan).

2.2 Procedure

Colorimetric method 0.5  ml MT standard solution (140 µmol L−1)and 0.5 ml CR-I solution, together with 1 mL HCl were transferred to centrifuge tubes and incu-bated at room temperature for 4  min. Then the mix-ture solutions were monitored at 400  nm by UV–vis spectrometer.

Fluorometric method 10 μL standard solution (10 µmol L−1) MT was mixed with 50 μL CR-II, 80 μL HCl and 920 μL ultrapure water, then were transferred to centrifuge tubes and incubated at 45 °C for 30 s. Finally, the resulting solutions were monitored at 540 nm emission with excita-tion at 410 nm by the fluorescence spectra.

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3 Results and Discussion

3.1 Principle of Colorimetric and Fluorometric Analysis

To investigate the sensing principle for MT, 2,3-Nda, ultraviolet and fluorescence spectrometry of MT, CR-I, the product of CR-I and MT were scanned, respectively. As illustrated in Fig. 1, MT (37.5 µmol L−1), CR-I (0.4 mmol L−1) and 2,3-Nda (0.4 mmol L−1) are no absorbance at

400 nm, while their product increases in absorbance at 400 nm. Meanwhile, the color of the mixture changed from colorless to yellow (Fig. 1 inset image). As depicted in Fig. 2, the fluorescence intensities of 2,3-Nda (9.52 µmol L−1), CR-II (9.52 µmol L−1) and MT (10 µmol L−1) were weak, but the fluorescence emission spectra of their prod-uct shows that it has a strong fluorescence at 540 nm because of a large conjugate structure. Meanwhile, the digital images of the different solutions exposed to a UV lamp were shown in Fig. 2 (insert), which further proved that the product has high fluorescence intensity.

Scheme 1 shows the possible mechanism for the determi-nation of MT by the simple colorimetric and fluorometric assay. 2,3-Nda and MT were catalyzed by FeCl3 in the acid condition and the mixture changed from colorless to into yel-low. It was found that the concentration of MT is directly proportional to the degree of yellow color. On the other hand, the product has a large conjugated system with strong fluo-rescence and can be applied to the fluorometric analysis with the excitation and emission wavelengths for 410 and 540 nm respectively. Based on the mechanism, we designed simple colorimetric and fluorometric assay for rapid detection of MT.

3.2 Optimization of sensing system

For colorimetry In order to improve the sensitivity for the determination of MT by colorimetry, the following factors were optimized: (a) reaction time and the concentration of HCl, (b) the concentration of Fe3+. The effect of time and HCl on the absorbance (Fig. 3a) shows that the absorbance almost reached its maximum value at 4 min and 6 M. And Fig. 3b indicates that the best result was gained when Fe3+ concentration was 0.05 mmol L−1. Figure 3c shows the maximum absorbance was obtained when the concentration ratio of Fe3+ and 2,3-Nda was 1:4, which was used for subsequent experiments.

For fluorometry Similarly with colorimetry, the same factors were optimized in order to obtain the optimal test condition for MT by fluorometry. As shown in Fig. 4a, the reaction rate is fast and the fluorescence almost reached its maximum at 30 s. From Fig. 4b, it was found that the higher the temperature is, the better the fluorescence quan-tum yield is. Consider the particularity of saliva samples, 45 °C was chosen as the reaction temperature. As can be seen from Fig. 4c, the maximum fluorescence intensity was gained with the concentration of Fe3+ at 8 × 10−5 mmol L−1. The volume of HCl has a greatly significant effect on fluorescence intensity. From Fig. 4d, we can con-clude that when the volume of HCl is 80 µL, fluorescence intensity almost reached its maximum. In the Fig. 4e, we can conclude that when the ratios of Fe3+ and 2,3-Nda was 1:4, fluorescence intensity reached stable and sensi-tive. Based on the optimal experimental results, we set the optimal conditions for the experiment to detect MT.

Fig. 1 UV–vis spectrum of 2,3-Nda (0.4 mmol L−1), CR-I (0.4 mmol L−1), MT (37.5 µmol L−1) and the product of the CR-I and MT. inset: digital image of CR-I (1), MT(2) and the product (3)

Fig. 2 The fluorescence spectrometry of CR-II (9.52 µmol L−1), 2,3-Nda (9.52 µmol L−1), MT (10 µmol L−1) and the product of the CR-II and MT. inset: the digital images of CR-II (1), MT (2) and the prod-uct (3) under the UV lamp

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Scheme 1 The detection mech-anism for MT by the colorimet-ric and fluorometric assay

Fig. 3 Factors affecting absorbance of a time and the concentration of HCl. The concentration of 2,3-Nda, Fe3+ and MT is 0.4 mmol L−1, 0.05  mmol L−1 and 35  μmol L−1, respectively. The concentrations of HCl from bottom to up were 1 M, 2 M, 3 M, 4 M, 5 M and 6 M

respectively. b The concentration of Fe3+. The concentration of 2,3-Nda, MT and HCl is 0.4 mmol L−1, 35 μmol L−1 and 6 M, respec-tively. c The ratios of Fe3+ (0.25 mmol L−1) and 2,3-Nda (0.5 mmol L−1), and experimental conditions was same as above

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Fig. 4 Factors affecting fluorescence intensity of time (a), the con-centration of 2,3-Nda, Fe3+ and MT is 9.52 μmol L−1, 0.08 mmol L−1 and 12  μmol L−1, respectively, the volume of HCl is 20  µL. b The effect of temperature. The concentration of reactant is same as (a) and the reaction time was 30 s. c The effect of the concentration of Fe3+ (80, 8, 8 × 10−1, 8 × 10−2, 8 × 10−3, 8 × 10−4, 8 × 10−5, 8 × 10−6,

8 × 10−7 and 8 × 10−8 mmol L−1). The concentrations of the other reactant are same as (b). d The effect of the volume of HCl. The condition was same as (b). e The effect of the concentration ratios of Fe3+ (10−4 mmol L−1) and 2,3-Nda (48 μmol L−1). The other condi-tion was same as (b)

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3.3 Linearity and limit of detection

For colorimetry As previously mentioned, the concentra-tion of MT is directly proportional to the degree of yellow color. To test the performance of this analytical method, we used the reaction system to detect MT under the optimum conditions. As shown in the Fig. 5a, the concentration of MT increased in direct proportion to the absorbance of the solution at 400 nm in the range of 2.5–37.5 µM. The linear equation was expressed as Y = 0.0152X − 0.0233 (μM) with the correlation coefficient (R2) of 0.998, and the limit of detection (LOD) is 1.288 μM (S/N = 3). Furthermore, the experimental results (Fig. 5b) show that the color change with decreasing concentrations of MT agrees with the previ-ous discussion.

For fluorometry At the same time, a linear regression model was constructed for MT with the use of the fluores-cence methods under the experimental conditions described above. The calibration parameters (Fig. 6a) indicate a good fit to the linear model. Figure 6b shows the sensitivity for MT by color change. It is found that the color of the mixture

solution still kept yellow under a UV lamp after the addition of MT in high concentrations. And when MT in low concen-trations was added, the color of the mixture became color-less. A linear calibration was obtained in the 0.01–0.1 µM concentration range, and the correlation coefficient reached 0.997 with the limit of detection concentration for 0.004 μM (S/N = 3), which shows the fluorescence assay is more sensi-tive than colorimetry naturally.

3.4 Test of selectivity

To test the selectivity of the assay, the control experiment was performed based on several potential interfering ions, such as carbohydrate and amino acids. We added the inter-fering substances to the reaction system in the presence of MT. The finally concentration of these compounds were 100 times to the standard concentration of MT. As shown in Fig. 7a, b, the influence of interfering substances on both absorbance and fluorescence intensity is very minor, dem-onstrating the high selectivity of the assay.

Fig. 5 a Sensitivity for deter-mination of MT (from down to up: 2.5, 7.5, 10.0, 12.5, 15.0, 17.5, 20.0, 22.5, 32.5, 35.0 and 37.5 μM), inset: linear calibra-tion plot for MT. b A digital image of reaction systems with different amounts of MT (from left to right: 2.5, 5.0, 7.5, 10.0, 12.5, 15.0, 17.5, 20.0, 22.5, 25.0, 27.5, 30.0, 32.5, 35.0, 37.5 and 40.0 μM)

Fig. 6 a CR-II incubated with a series of various concentrations of MT standard solution (from down to up: 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 and 0.1 μM), inset: linear calibration plot for MT. b A digi-

tal image of reaction systems with different amounts of MT exposed to a UV lamp (from left to right: 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07 and 0.10 μM)

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3.5 Real samples detection

To verify the practicability of the two developed MT assay, the human saliva samples were provided by two healthy vol-unteers and were tested by the proposed system. The volun-teers were not allowed to eat or drink for 2 h before collect-ing saliva to avoid interfering with the experiment. Saliva samples were collected at 24:00 and need to be frozen for 24 h. After thawing, the samples were treated with 9000 rpm centrifugation for 40 min. Supernatant was retained and diluted with equal ethanol. Then the mixed solution was treated with 9000 rpm centrifugation for another 15 min. Finally supernatant was collected for analysis. Standard addition method was applied to get the recovery of two methods. As summarized in Table 1, the recovery for MT of two methods were in the range of 82.89–113% and 80–110%, respectively with RSD (n = 3) for less than 5.9% and 4.0%, respectively. The results demonstrated that the two proposed methods can be applied to detect melatonin in saliva.

4 Conclusions

In summary, a simple and low-cost colorimetric and fluo-rometric assay for MT was proposed by use of 2,3- naph-thalenedialdehyde as color reagent. The method show sen-sitivity and high-selectivity toward MT with the limit of detection of 0.004 µM, and has been applied to assay MT in the human saliva samples with the satisfactory results. The proposed assays are fast, sample and low cost, and the required instruments are easy to get. Above advantages prove the proposed colorimetry and fluorometry can be applied to fast clinical examination of melatonin in human saliva.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (1765015, 21808099) and the Science and Technology Innovation Platform Project of Jiangxi Province (20192BCD40001).

Fig. 7 The selectivity of colorimetric method (a) and fluorometric method (b). For colorimetry: The concentration of MT was 20 µM, and the other interfering substances were 2 mM. For fluorometry The concentration of MT was 0.05 µM, and the other interfering substances were 5 µM

Table 1 Determination of MT spiked in human saliva by colorimetric and fluorometric assay

Sample Colorimetry Fluorometry

(n = 3) Added (µM) Found (µM) Recovery (%) RSD (%) Added (µM) Found (µM) Recovery (%) RSD (%)

Sample1 0 0 – – 0 0 – –10 11.3 113 4.2 0.02 0.016 80 1.720 20.6 103 5.2 0.05 0.0509 101.8 0.735 29.01 82.89 5.3 0.09 0.0823 91.44 1.4

Sample2 0 0 – – 0 0 – –10 8.9 89 5.9 0.02 0.018 90 2.020 21.03 107.5 2.1 0.05 0.055 110 3.635 32.87 91.42 5.7 0.09 0.0923 102.56 4.0

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