Paper microfluidic device using carbon dots to detect glucose and lactate in saliva samples

Highlights

• Easy and non-invasive sample collection using saliva.

• A low-cost sample treatment using a cotton-paper-syringe filtration system.

• LODs as low as 10⁻⁶ mol L⁻¹ and 10⁻⁷ mol L⁻¹ were achieved.

• Simple, fast, low cost and highly sensitive method to analyze glucose and lactate.

Abstract

Bioanalyses are commonly performed with blood or serum samples. However, these analyses often require invasive and painful blood collection using a needle or finger pricking. Saliva is an alternative and very attractive biological medium for performing clinical analyses, since it contains many types of clinically relevant biomarkers and compounds. Its collection is straightforward and can be achieved in a non-invasive and stress-free way. However, the analytes are frequently present at low concentrations, while the viscosity of whole saliva hinders its analysis using paper devices, especially those with multiple layers (3D-μPADs). This work explores the use of a simple, fast, and low-cost saliva sample pretreatment using a cotton-paper-syringe filtration system, allowing the analysis of saliva samples using multilayer paper devices. The proposed methodology employs the oxidation of glucose and lactate, catalyzed by specific oxidase enzymes, producing hydrogen peroxide. The detection is based on the fluorescence quenching of carbon dots in the presence of hydrogen peroxidase. The concentrations of the analytes showed good linear correlations with the fluorescence quenching, with LODs of 2.60 × 10⁻⁶ and 8.14 × 10⁻⁷ mol L⁻¹ for glucose and lactate, respectively. The proposed method presented satisfactory intra-day and inter-day repeatabilities, with %RSD values in the range 3.82–6.61%. The enzymatic systems proved to be specific for the analytes and the matrix had no significant influence on the glucose and lactate determinations. The proposed methodology was successfully applied to saliva and serum samples and was validated using certified material.

Introduction

Since paper was first used as a microfluidic platform in 2007 [1], the microfluidic paper-based analytical device (μPAD) has become a powerful tool for applications in different fields including environmental analysis [2], food analysis [3], and clinical diagnostics [4]. Essentially, these paper-based devices are paper platforms containing hydrophilic channels that are delimited by hydrophobic barriers constructed using wax-printing, photolithography, stamping, or dipping, among other methods [5]. The μPAD combines the advantages of a paper platform, such as low cost, easy availability, flexibility, hydrophilicity, light weight, biocompatibility and power-free fluid transport, with microfluidic control of fluid transport, small sample volume, and low reagent usage [6], [7], [8].

Colorimetry and electrochemical methods are the most commonly used detection techniques for analysis using paper devices, because they allow the use of simple, easy-to-use, low cost, and portable instrumentation. The utilization of fluorescence as a detection method for paper devices is still limited, compared to other techniques [9], [10]. It offers advantages similar to those of colorimetry, providing analytical results by direct visual read-out. Furthermore, it can improve the sensitivity and selectivity of clinical analyses, due to its low detection limits [11].

Fluorescence is an attractive detection method for the development of μPAD techniques. However, the lack of available fluorescence reactions still limits its application. One option is to exploit the use of carbon dots (CDs), which are carbon nanoparticles that have attracted considerable attention due to their unique properties. Compared to conventional semiconductor quantum dots (QD), CDs present advantages such as low toxicity, good water solubility, high biocompatibility, simple and inexpensive synthesis routes, abundant sources of precursors, and rich contents of functional groups, facilitating surface modification or functionalization. At the same time, they possess QD properties including high photostability, resistance to photobleaching, and good photoluminescence [12], [13]. Consequently, they have been employed in bioimaging [14], biosensing [15], catalysis [16], drug delivery [17], and sensors for organic compounds [18], [19], [20] and metals [21], [22]. The introduction of CDs increases the potential of new fluorescence methods using paper platforms, especially for clinical applications, because this association can combine the low detection limits of fluorescence methods with the simplicity and low cost of paper devices, enabling the creation of ASSURED (Affordable, Sensitive, Specific, User-friendly, Robust and rapid, Equipment-free, and Deliverable to those who need them) point-of-care (POC) testing [23].

Measuring glucose concentration is one of the most frequently performed procedures in hospitals and laboratories, since glucose metabolism is crucial to life [24]. Diabetes mellitus and hypoglycemia are clinical conditions associated with abnormal glucose metabolism, with the International Diabetes Federation estimating that 578 million people worldwide will have diabetes by 2030, reaching 700 million by 2045 [25].

Lactate is an intermediate in anaerobic carbohydrate metabolism. At high concentrations in blood, it decreases the pH and can cause muscle damage [26]. Lactate concentrations above 5 × 10⁻³ mol L⁻¹ and pH below 7.25 are indicative of significant lactic acidosis [24]. Lactate measurement provides a less invasive way to control diabetes in patients and to diagnose type I glycogen storage disease, which decreases the ability of the liver to produce glucose-6-phosphate from glucose [27].

The commonest matrix used to determine glucose and lactate is blood, especially for glucose, given the widespread availability of commercial glucometers. However, this analysis requires an invasive and painful blood collection using needle or finger pricking, which can be stressful if performed several times daily. Saliva is an alternative and very attractive biological matrix for performing clinical analysis [28]. Different to blood sample collection, saliva sample collection is straightforward and can be performed non-invasively and without causing stress, allowing repeated sample collections within short time periods, performed by modestly trained operators [28], [29]. Consequently, increasing numbers of studies concerning saliva diagnostics have been published for analytes including glucose [4], [30], lactate [26], nitrite [4], [31], tyrosine [32], and hormones [33].

The main component of saliva is water (99.5%), with 0.3% of proteins and 0.2% of inorganic compounds and trace substances [34]. Other components found in saliva are shaded squamous epithelial cells, white and red blood cells, microorganisms, and high molecular weight mucins [31], [35]. The presence of proteins, large glycoproteins, and cells significantly influences the viscosity of whole saliva [31], [34], hampering its analysis using paper devices, especially when multilayer devices (3D-μPADs) are employed. It is possible to minimize the effect of viscosity by precipitating proteins with chemicals, ultrafiltration, or pH adjustment, but these protocols can influence the compounds of interest or the composition of saliva [34], while requiring the use of sophisticated equipment that may not be available in situ or in remote regions where resources are limited [31]. Hence, easier and simpler protocols should be used in the development of low-cost paper devices that can be used by a modestly trained operator.

In this work, a new, simple, and low-cost methodology was developed using a 3D paper-based analytical device for the determination of glucose and lactose in saliva and serum samples, with fluorescence detection. The method employs a simple, fast, and low-cost sample treatment using a cotton-paper-syringe filtration system, enabling 3D-μPAD analysis of saliva samples. The combination of enzymatic reaction with fluorescence detection using carbon dots, produced by a one-pot microwave-assisted synthesis, resulted in a specific and sensitive methodology with low limits of detection. The methodology described here is a valuable analytical tool for the monitoring of glucose and lactate levels.