Fig 2: Distribution of BDNF and its receptors in the zebrafish. (a) Tissue distribution of bdnf mRNAs in zebrafish. Quantitative analysis of mRNA expression was performed by RT-qPCR considering β-actin as reference gene. Data are expressed as mean + SEM (n = 6), relative to the tissue with the lowest mRNA expression. (b) Full-length Western blot image showing BDNF protein in zebrafish tissues (n = 2). Protein molecular weight (in kDa) is shown in figure. (c–h) Representative sections of zebrafish gut (c–e) and liver (f–h) showing BDNF immunofluorescence (green). A magnified image of representative cells immunopositive for BDNF is shown in a square inset for both foregut and liver. In insets, nuclei are stained blue (DAPI). No or small immunoreactivity was detected in negative (d,g) or preabsorption (e,h) controls. Scale bars are indicated in each image. (i–k) Tissue distribution of mRNAs encoding BDNF receptors in zebrafish. Data obtained by RT-qPCR are expressed as mean + SEM (n = 6), relative to the tissue with the lowest mRNA expression. Ac absorptive cell, BDNF brain-derived neurotrophic factor, Ep epithelium, Lp lamina propria, p75ntr neurotrophin receptor p75, trkb tropomyosin receptor kinase B.

Fig 3: Schematic illustration of the ultra-sensitive detection platform for BDNF using Au@SiO2 and enzymatically synthesized QDs.

Fig 4: The quantitative detection of BDNF using GNPs and Au@SiO2. (a) Fluorescence intensity profiles for different concentrations of BDNF using Au@SiO2: (I) 1 μg/mL, (II) 100 ng/mL, (III) 10 ng/mL, (IV) 1 ng/mL, and (V) 0 ng/mL (Control). (b) Fluorescence intensity profiles for detecting BDNF using identical concentrations of GNP: (I) 1 μg/mL, (II) 100 ng/mL, (III) 10 ng/mL, (IV) 1 ng/mL, and (V) 0 ng/mL (control). (c) A comparative fluorescence detection analysis between GNPs and Au@SiO2 across varying BDNF concentrations, represented as endpoint relative fluorescence unit (RFU) values. (d) A linear correlation analysis of the endpoint fluorescence values for BDNF detection using Au@SiO2; a strong positive correlation with BDNF concentration was observed (R2 = 0.9384).

Fig 5: Confirmation of a QD formation around Au@SiO2 following BDNF detection via an elemental analysis. (a) TEM image showing BDNF detection in regard to Au@SiO2. ALP-conjugated antibodies specific to BDNF were attached, and QDs were formed through an enzymatic reaction on the nanoparticle surface. (b) An elemental ratio graph obtained from an EDS analysis showing the following weight percentages: Si 1.96%, S 10.72%, Cd 37.07%, and Au 50.25%. (c–f) Elemental distribution images showing the presence of (c) sulfur (S), (d) cadmium (Cd), (e) gold (Au), and (f) silicon (Si).