Preparation of carbon dots with orange emission for Cr(Ш) detection and cellular imaging

Chromium and its compounds, which were widely applied in metallurgy, electroplating, coating and battery industries, have been discharged into the water environment with an increasing amount in recent years [1]. Cr(Ш) is one of the main ionic forms of chromium element in the environment. For the human health, Cr3+ has been identified to be essential to maintain the normal carbohydrate metabolism [2]. However, under the excessive condition, it will hurt the chromosome structure, respiratory system and even lead to cancer [3]. So, it would be of great significance to develop efficient methods to detect Cr(Ш). At present, the main methods for the determination of Cr(Ш) are atomic absorption spectrometry, atomic emission spectrometry and X-ray fluorescence spectrometry, etc. [4, 5]. However, there are some problems for these methods, such as high cost, timeconsuming and complex operation, which limit their application and development. The detection of metal ions based on fluorescence analysis has the advantages of simple operation, high sensitivity and good selectivity [6]. Therefore, the establishment of new fluorescence methods for Cr(Ш) detection has as great value both on theory and practice [7]. Fluorescent carbon dots (CDs), a fascinating star of “zerodimensional” nanomaterials, have received huge attentions owing to their physicochemical properties, such as good water solubility, low biological toxicity, flexible modification [8]. Because of their particular advantages, versatile carbon sources and synthetic routes have been developed to produce CDs for vast applications, such as bioimaging, drug loading and photoelectric materials, etc. [9, 10]. However, most of the reported CDs could emit fluorescence in the blue or green region under excitation with a few exceptions [11]. As we know, the fluorescent nanomaterials with long wavelength absorb/emit performance might be more popular in the bio-related applications, because they could effectively avoid the influence of autofluorescence and possess excellent optical penetrability in biological structure [12]. As a result, it is of great significance to produce CDs with long wavelength emission for use, especially, biological applications. In this research, a new type of orange emission carbon dots (OCDs) was prepared by hydrothermal method using 1,2,4-