Bacterial infections show a serious threat to human lives and the development of effective medicines to disinfect bacteria are in great demand. 1 Various antibiotics have been used for treating bacterial infections, but the overuse of antibiotics causes other problems such as side effects and drug-resistant issues. 2 The nanomaterials including antimicrobial polymers, 3 metal nanomaterials, 4, and carbon nanomaterials 5,6 have been used as alternatives to classical antibiotics. 7 Both drug-resistant and toxic problems are relieving. 8 Recently, CDs 9 and nanoclusters (NCs) 10 are well applied for combating bacterial infections because they are biocompatible, active, and can be easily cleaned by circulations due to the ultra-small sizes. 11,12 Especially, researchers have found that CDs show excellent free radical scavenging ability, which can be stronger than many traditional anti-infection drugs. 13–15 However, these ultra-small antimicrobials also suffer poor stability due to the larger oxidative surface area. 16 It is highly desired to develop more effective antibacterial agents for combating bacterial infections for long-term use.
To meet the demand for practical applications, the antimicrobials should have the following characteristics: a) Excellent stability remains unchanged for a certain time in an ambient environment; b) Excellent biocompatibility and low toxicity: c) high antibacterial activity. The larger nanomaterials are more stable, but relatively weaker antibacterial activity tends to be exhibited due to the smaller active surface area. Considering both the weakness of small and large nanomaterials, we report the assembly of the small CDs into large CDFs by simply adding polyethyleneimine (PEI) (Fig. 1). CDs were not fused but kept their morphology as building blocks. Therefore, the entire CDFs showed larger sizes but demonstrated more excellent stability without losing the active properties of CDs. Further, we found the CDFs displayed enhanced antibacterial activities against both Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) compared to CDs, indicating their broad-spectrum antibacterial performance. This work suggests the assembly of small CDs into large CDFs not only enhances the stability but also magnify the antibacterial activity.
The following are available online at www.mdpi.com/xxx/s1, Figure S1: TEM (a) and AFM (b) of the as-obtained CDs; Figure S2: (a) UV-vis, (b) the fluorescence excitation and emission spectra of the CDs; Figure S3: XPS survey (a) and FTIR of the as-obtained CDs; Table S1: Lifetime of the CDs and CDFs.