Smartphone-based fluorometer for pH detection using green synthesized carbon dots

Abstract

As a group of heavy-metal-free fluorescent nanomaterials, carbon dots (CDs) have recently received a growing interest as they have high optical absorption activity and are chemically stable, biocompatible, and less toxic. In this paper, the green synthesis of stable CDs by hydrothermal and pyrolysis methods from Parmigiano-Reggiano is performed. An intrinsically emitted fluorescence results from simultaneous generation and the surface passivation of CDs by the mentioned techniques. The responses of hydrothermal CDs to pH occur by emission wavelength at 430 nm under 365 nm excitation. When the pH is increased from 2–12, its emission fluorescence intensity will be reduced. The pH was also analyzed real time by developing a smartphone-based portable recognition platform consisting of an instrument and a self-designed APP called “Image Analyser”. The fluorescence color change of CDs caused by the pH was recorded by the camera of the smartphone and color of pictures was analyzed by APP. The results of both methods are similar, so the CDs based recognition platform are highly practically and efficient tool as they could analyze pH with outstanding functionality, as examined in this study in different samples.

Introduction

The main fluorescent substances are organic dyes, semiconductor quantum dots (QDs), rare earth doped nanoparticles, and metal nanoclusters [1], [2], [3], which are applied limitedly. They are less stable, seriously toxic, or very costly. Nanomaterials demonstrate exciting characteristics [4], [5] and carbon dots (CDs) as a kind of fluorescent nanomaterial have received growing attention. They have high optical absorption activity and are chemically stable, biocompatible, and less toxic [6]. Such excellent specifications render these materials potential choices for a variety of interesting utilization, including bioimaging, therapeutic diagnostics, catalysis, and photovoltaic instruments [7], [8], [9]. Despite the preparation of fluorescent CDs with copious attempts, additional studies on CDs face plenty of issues scientifically. The current synthetic ways of fluorescent CDs are grouped into two categories of bottom-up and top-down [10], [11], with the former procedure being mostly dependent on thermal decomposition, combustion, and dehydration of appropriate molecular precursors [10]. The latter method was typically used as a post-treatment for nanocarbon destroyed from different greater carbon structures [11]. In recent years, multicolor emission CDs have been synthesized and applied with considerable advancements [12], [13] but the improvement of facile, green and controllable synthesis of CDs is still required.

The method of analyzing fluorescence responds rapidly, has a high spatial resolution, and is safe for handling remotely, which is used for detecting metal ions with high sensitivity and selectivity, pH, temperature, and so on [14]. There is a recent report about preparing CDs and utilization in small biomolecules and analyzing pH [13]. CD has a high sensitivity to pH because the functional groups, particularly carboxylic acid groups, are ionized on the surface. CDs also present less toxic activity and better biocompatible property. The presence of abundant hydrophilic surface functional groups in CDs makes them simply dispersible in hydrous media with no need for additional modification of the surface.

The ever-increasing development in hardware and software has made smartphones equivalent to mini computers with internal memory, operating systems, and high quality cameras, whereby granting opportunities to develop analytic sensing systems and their utilizations according to previous reports [14], [15]. These can typically be exemplified as the optical-sensing platforms for colorimetry [16], fluorimetry [17], resonance light scattering [18], chemiluminescence [19], electrochemiluminescence [20], data analysis, and control interfaces [21]. Clearly, a smartphone-based analytic instrument is advantageous in terms of movability, less power use, low cost, and convenient use in location and exchanging data for field analyses [22], [23], leading to the rapid development of smartphone-based fluorescent recognition instruments [24], [25]. A camera is generally used to capture the fluorescent image of the luminescent area. The image brightness is utilized as the analytic signal. An excitation light source with high intensity and stability is required for obtaining a dependable fluorescent signal. Ordinary light sources, e.g., laser pointer or LED flashlight even ambient light are applied depending on considerations for field analyses [18], [26]. Therefore, the drift in light intensity is not likely avoidable in field analyses, because of the battery power use and the ambient temperature variation. Additionally, the smartphone should be a separable component in the moveable recognition instrument to maintain basic communication functionality. Therefore, when the smartphone changes or the camera restarts, eliminating the impact of variations in focus is also necessary. Lastly, convenient quantitative field-analysis is achievable real-timely by the digital image colorimetry based on smartphones in comparison to conventional analytic techniques that require rather complex equipment (Table 1).

Here, CDs were synthesized by designing one-step hydrothermal and pyrolysis routes with the Parmigiano-Reggiano as the precursor. The CDs displayed blue fluorescence with maximal emission at 430 nm and high quantum yield ($\sim$13%). Hydrothermally CDs have small-sized (2–6 nm) with proper dispersal in water. The manufactured hydrothermal CDs show responses to pH. Moreover, the pH was analyzed real time by developing a smartphone-based moveable recognition platform consisting of an appropriate instrument and an APP program. The fluorescence color variation of CDs caused by the pH was recorded by the dark box device attached camera of the smartphone. The curve based on the color of pictures was fixed by a self-designed APP called “Image Analyser”. Scheme 1 exhibits the schematic of the synthesized CDs and applications of hydrothermally CDs for pH detection by smartphone.