Gold nanocluster-based fluorescence sensing probes for detection of dipicolinic acid.

Bacillus spp. are spore-forming bacteria, and some of them, including Bacillus cereus and Bacillus anthracis, are pathogens. Dipicolinic acid (DPA) has been recognized as a biomarker for spore-forming bacteria. Thus, developing rapid sensing methods to spot the presence of DPA in suspicious samples is significant. In this study, we employ complexes of glutathione-capped gold nanoclusters (Au@GSH NCs) with Cu2+ as sensing probes against DPA. Au@GSH NCs possess orange-reddish fluorescence. However, their fluorescence is significantly quenched in the presence of Cu2+. In the presence of DPA, the fluorescence of Au@GSH NCs can be restored because DPA can easily remove Cu2+ on the NCs through chelation based on the high formation constant (log K = 7.97) between Cu2+ and DPA. Therefore, on the basis of this fact, Au@GSH NC-Cu2+ complexes are used as turn-on fluorescence probes against DPA. Unlike most of the existing sensing methods, the developed Au@GSH-Cu2+-based sensing method is not affected by the presence of phosphates, which can be commonly found in real samples. The limit of detection of using the developed sensing method toward DPA can reach as low as ∼19 nM. In addition, we also demonstrate the feasibility of using the developed sensing method for detection and quantification of DPA in soil samples and B. cereus spore lysates.

Gold nanoparticle-based colorimetric sensing of dipicolinic acid from complex samples.

Dipicolinic acid (DPA) can cause neurotoxicity and is abundant in bacterial spores. Although analytical methods have been reported for DPA detection with high sensitivity, their selectivity toward DPA is declined greatly in the presence of phosphates in the samples. In this study, we developed an approach for DPA detection that is not affected by the presence of phosphates. A colorimetric method based on the use of gold nanoparticles (AuNP) complexed with Ca2+ as sensing agents was explored for DPA detection. Calcium ions and glutathione-capped gold nanoparticles (AuNPs@GSH) can easily form complexes (Ca2+-AuNP@GSH) through GSH-Ca2+ chelation, leading to the aggregation of AuNPs@GSH. The aggregation resulting from the complexes of AuNPs@GSH and Ca2+ can be reversed with the addition of DPA owing to the high formation constant (log Kf = 4.4) between DPA and Ca2+. Furthermore, the color of AuNPs@GSH changes from red to purple when complexed with Ca2+, returning to red upon addition of DPA. The limit of detection of this sensing method toward DPA was estimated to be as low as ~ 2 µM. The feasibility of using the sensing method for quantitative detection of DPA in soil and Bacillus cereus spore samples was also demonstrated. Graphical abstract A AuNP-based colorimetric sensing method against dipicolinic acid is developed.

Bright carbon dots as fluorescence sensing agents for bacteria and curcumin.

Carbon dots (C-dots) are fluorescent nanomaterials that possess good photostability and low toxicity. They have been used as sensing probes and bioimaging agents for a variety of biological species. Numerous methods are available to generate C-dots. Nevertheless, simple and straightforward synthesis methods must be explored for the synthesis of C-dots from inexpensive, natural sources. In this study, we developed a simple method to generate C-dots from inexpensive chicken egg whites through a one-step heating reaction. The size of the generated C-dots was 3.3±0.4nm, and the quantum yield of the C-dots was as high as ∼43%. The as-prepared C-dots can be used as multicolor labeling agents for bacteria such as Staphylococcus aureus and Escherichia coli. Furthermore, the generated C-dots can be used as Förster resonance energy transfer sensing probes for curcumin, which is an active ingredient of turmeric and medicinal pigment. The feasibility of using the C-dots as selective sensing probes to determine the amount of curcumin from complex turmeric powder and condensed turmeric tablets is also demonstrated.