Cu is a trace element that plays a crucial role in the proper functioning of organs, however, excessive Cu ions (Cu2+) may cause toxic health effects. Hence, distinguish of Cu2+ concentration in water is critically important for modifying the human health and environmental contamination [[1], [2], [3]]. Several efficient methods such as inductively coupled plasma atomic emission spectrometry, high-performance liquid chromatography, and electrochemical sensing have been used for Cu2+ detection. These methods always possess some limitations including time-consuming, expensive instruments-dependent, and complexity [[4], [5], [6]]. Thus, a colorimetric sensing scheme with low cost, simplicity, ease of operation, and rapid response is of importance. In this regard, Ag nanoparticles (NPs) based colorimetric method in the field of metal ions sensors have attracted special interest because even a small change in the size, elemental composition or shape of NPs can lead to tunable, rapidly, and visible optical-electrical properties [7,8]. Ag NPs are widely used in water treatment, agriculture, medicine, drug delivery, cosmetics, electrocatalysis, photocatalysis, etc. [[9], [10], [11], [12], [13], [14], [15], [16]]. However, the serious particle aggregation caused by the high specific surface energy of Ag NPs poses a barrier to the commercial application.

Accordingly, Ag NPs can be functionalized with biopolymers and other molecular ligands to enhance the stability, sensitivity and selectivity due to their interesting naturally-based characteristics [[17], [18], [19]]. The use of many biopolymers including cellulose, gelatin, agarose, chitosan, and dextran as templates for the fabrication of Ag NPs based nanocomposites can give these nanocomposites the featured properties, such as photocatalytic, magnetic, flame retardant, electrical conductivity, high tensile strength, elasticity, extensive surface [20]. For instance, a highly homogeneity and stability of Ag-agarose nanocomposite can be successfully fabricated through the interaction between the rich hydroxyl (-OH) functional groups in agarose and Ag NPs [21,22]. Preferential coordination of carboxyl (-COOH) and amino (−NH2) in amino acids with Cu(II) has been well-documented and experimentally demonstrated in various nano-sensing systems [[23], [24], [25]]. Chitosan is a biodegradable, biocompatible, cheap, environmental-friendly alkaline polysaccharide. Most notably, the abundant active functional groups of -NH2 and -OH in structure made chitosan to be efficient adsorbent for heavy metal ions, and the electrons in lone pair also imposed -NH2 group strongly coordinated with metals [26]. Chitosan functionalized gold nanoparticles assembled on sulphur doped graphitic carbon nitride can be used as a new platform for colorimetric detection of trace Hg2+ [27,28]. However, chitosan has a poor mechanical strength and structure unstability in solutions which hinders its application in rapid heavy metal ions testing strip. To address these shortcomings, chitosan was generally chemically modified, but meanwhile, the reduction of free –NH2/-OH groups results in a lower density of binding sites for heavy metal ions [[29], [30], [31], [32]]. Furthermore, most of chemical modification agents are always extremely toxic, then the focus has shifted substantially towards other crosslinkers of natural origin in recent years [[33], [34], [35], [36], [37]].

Amino acids are amphoteric molecules due to their unique zwitterionic nature of −COOH and − NH2 groups coexisting, and the functional groups can interact with polar groups on the chitosan by electrostatic reaction and hydrogen bonds, which may be expected to act as acid regulator and structure-directing agent to induce the formation of chitosan gel structure [[38], [39], [40]]. And compared with other conventional chemical modification agents, amino acids have the potential advantages of biodegradable, green and environmentally friendly [41,42]. Furthermore, double network hydrogels can be developed to improve the mechanical properties via the combination of the two interpenetrating polymer networks by tuning the intramolecular interactions [[43], [44], [45], [46]].

Inspiration of this, a facile method for the green synthesis of chitosan/glutamic acid/agarose/Ag (Chi/GA/Aga/Ag) nanocomposite hydrogel was obtained via in situ reduction of Ag ions during the crosslinking process of chitosan-agarose double network hydrogels. Glutamic acids was substituted for toxic chemical modification agents to induce the formation of chitosan/glutamic acid hydrogel. And agarose was introduced to form the chitosan-agarose double network hydrogels to enhance the mechanical strength of the Chi/GA/Aga/Ag composite hydrogel. The interaction between the rich -NH2, -COOH, and -OH functional groups in both agarose, chitosan, and glutamic acid with Ag NPs can effectively control the growth and dispersion of Ag NP in the Chi/GA/Aga/Ag composite hydrogel. The Chi/GA/Aga/Ag composite hydrogel can be directly used as a visual test strip of the Cu ions, and a proposed mechanism for the Cu ions detection was provided.