Vitamin B12 (VB12), also called as cyanocobalamin, plays a vital role in normal metabolism of enzymes, lipids and carbohydrates, nerve cell maintenance, DNA synthesis and red blood cell formation [1–4]. For adults, the recommended dietary allowance of VB12 are about 0.40–2.80 mg per day . The excessive VB12 can resulted in liver disease, asthma, neurotoxicity and kidney disorders. Moreover, deficiency of VB12 may lead to significant public health problems like neurological degeneration, memory loss, pernicious anemia and increases the risk of heart disease . Vitamin B12 is one of the essential water soluble ‘‘B complex vitamins’’ that cannot be produced by body cells, it can also be taken from foods like milk, meat, egg and pharmaceutical supplements in various dosage forms daily . As recommended by a healthcare practitioner, the deficiency of VB12 can often be reversed by consuming supplements or by fortified foods . At present there is great attention in accurately assessing the total dietary intake of vitamins B12 from all sources, especially vitamin supplements (tablets or injections). Therefore, there is an urgent need to develop an accurate, fast and portable method for on-site quantification of VB12 at the quality control of pharmaceutical companies.
At present, most quantification of vitamin B12 are conducted using high performance liquid chromatography (HPLC) , enzyme-linked immunosorbent assay (ELISA) , chemiluminescence , liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) , capillary electrophoresis and electrochemical methods [7, 13]. Generally, these laboratory-based methods are expensive both in reagent and services and requires professionally trained operators . Also, another limitation of laboratory-based methods is their time-consuming properties, which leads to factory production line wait for several hours for the test results in order to release the high-quality products. Therefore, the development of an efficient, affordable and simple nanosensor-based systems with sufficient analytical performance for accurate and fast monitoring of vitamin B12 content is necessary for the evaluation of supplement quality.
With the progress of quantum dots as advanced nonomaterials, they are considered as promising fluorescence sensing platform for quantification of VB12 [15, 16]. Compared with conventional semiconductor quantum dots, carbon-based quantum dots (CQDs) offer benefits such as highly tunable photoluminescence, chemical inertness, small size and photo-bleaching resistance and also low toxicity and biocompatibility [17, 18]. Up to now, among developed nanosensor for quantitative detection of VB12, fluorescence-based nanosensor using quantum dots have been reported more than the others [5, 8, 19]. It should be noted that, most of the developed fluorescent assays of VB12 for many applications are still conducted with expensive instruments in laboratory and are not suitable for fast on-site detection of analytes.
In recent years, smartphone-based detection system integrated with bio/chemical sensors has shown promising progress in point of care (POC)/ Point of use (POU) testing approaches for various analytes. Because they are portable, ubiquitous and also produce rapid and simple quantitative results that can be interpreted by untrained personnel especially in resource-limited areas [14, 20–23]. To-date, one research teams have been reported smartphone platform for POC quantification of VB12 in blood using immonuassay-based lateral flow biosensors . These biosensors fabrication needs complicated antibody preparation technology and test strip assembly skill, which is not easy to achieve. Furthermore, in the present study, a novel nicotinamide-functionalized CQDs nanosensor were designed as fluorescence and colorimetric probes for simple and portable detection of VB12 simultaneously. NA-CQDs were synthesized by one-pot microwave-assisted hydrothermal method emploing nicotinamide as new sources of functionalized material on carbon quantum dots for the first time. Moreover, we have try to make new nanosensor more user friendly and portable by developing in-house android-app on smartphone. But to our knowledge, no such work has been reported so far by the smartphone integrated with NA-CQDs nanosensors for the on-site monitoring of VB12 in pharmaceutical supplements. The NA-CQDs nanosensor exhibited dual response via both fluorescence and colorimetric change for on-site monitoring of VB12, simultaneously. In the RGB-based (red, green and blue color space) software, while the image is captured by the camera, simultaneous analysis can be done by the in-house developed android app and the quantitative results are displayed on the smartphone. Therefore, the portable smartphone-assisted colorimetric reader is promising to become a sensitive tool for quantitative colorimetric measurement and accurate on-site analysis of VB12 in many applications.