Introduction

Gold nanoparticles (AuNPs) represent one of the most promising materials for microbial sensing and diagnostics, mainly due to their unique optical properties [1]. A variety of methods have been proposed for the decoration of AuNPs for tagging microorganisms based on both chemical linkage or physical adsorption of specific ligands, including antibodies, oligonucleotides, aptamers, or phage-displayed polypeptides [2,3,4,5]. Nevertheless, there has been an emergence for the screening of microorganisms in different environments, and the availability of low-cost, functional AuNPs of broader applicability would be beneficial. Given the high negative charge of microbial cell surfaces, the use of electrostatic interactions to unspecifically tag microbial cells seems appealing. The use of poly-L-lysine (PLL), a highly biocompatible, cationic polypeptide of natural origin [6], constitutes an attractive alternative to obtain cationized AuNPs. Incidentally, in recent years, various peptide molecules, including PLL, have been reported to act as reducing agents during the synthesis of metal nanoparticles [7,8,9]. PLL has also been proposed for the immobilization of microorganisms onto inorganic materials, where attached cells can be used for capturing biomolecular ligands or testing antimicrobial agents [10, 11]. Noteworthy, the binding of PLL to bacteria seems to be discriminatory, as distinct degrees of attachment have been found for different cell surface structures [12, 13]. Therefore, a combination of PLL-binding activity with the optical sensing capabilities of nanogold appears very attractive for microbial sensing. Optical features of AuNPs, as used for sensing purposes, arise from the surface plasmon resonance (SPR) excitation, a phenomenon allowing the characterization of surfaces through various techniques, such as SPR-induced color changes or surface-enhanced Raman scattering (SERS) [14, 15]. It has been established that for these techniques to be exploited, nanoparticles must fulfill specific parameters namely sizes above 20 nm [16] and constructs allowing a 10-nm maximal distance between a metal and the tagged analyte for SERS [15, 17]. In this work, we developed a simple, one-pot, highly reproducible and scalable method to obtain functional AuNPs through reduction and functionalization with PLL. This method enables the production of spherical particles of uniform coating thickness, colloidal stability over time, and structural features that are suitable for optical sensing. Furthermore, the potential of the new construct to tag E. coli, leading to differential optical signals, was herein demonstrated.

Experimental

Synthesis

Cationic gold nanoparticles (PLL@AuNPs) were synthesized through a one-pot method, using PLL as a reducing and stabilizing agent. All the reagents were from Sigma-Aldrich (St Louis, MI, USA). The synthesis was carried out in a total volume of 10 ml, using 500 μl of 2.5 mM HAuCl4∙3H2O (≥ 99.9%). The gold precursor was diluted in distilled water and allowed to stir at boiling temperature (95–100 °C). Then, 450 μl of a PLL hydrochloride solution (5 mg/ml, mol wt 15,000–30,000) was added, and the reaction was allowed to proceed for a 10-min period. The formation of particles was followed by color change, from yellow to colorless, then to wine red. After 10 min, the reaction was stopped by ice-quenching. Finally, nanoparticles were centrifuged at 13,000 rpm for 30 min in order to remove the remaining free PLL.

Characterization techniques

UV-visible spectra were measured using a Genesys 10S UV-Vis spectrophotometer (Thermo Scientific, Waltham, MA, USA). The X-ray diffraction (XRD) analysis was performed using an Ultima IV diffractometer (Rigaku Co., Japan) with a Cu-kα radiation source. Transmission electron microscopy (TEM) analyses were carried out in a JEM 1010 (JEOL, Ltd., Japan) at 80 kV, by loading 5 μl of samples onto carbon coated copper grids, and staining with 2% uranyl acetate. The particle size and zeta potential measurements were performed using a Zetasizer Nano ZS (Malvern Panalytical, Ltd, UK). FTIR analyses were carried out in a Spectrum Two FT-IR Spectrometer (PerkinElmer, Inc., Waltham, MA, USA).

E. coli growth and labeling

E. coli XL-Blue was grown in LB nutrient broth at 37 °C under constant agitation. For SERS analyses, 50 μl of an overnight culture was inoculated in 5-ml LB medium at 37 °C until exponential growth phase. Then, cells were collected by centrifugation at 4 °C, 4000 rpm for 5 min, washed thrice with sterile saline solution (0.9% NaCl), and resuspended in 1 ml before labeling. Afterwards, various volumes (250, 500, 1000 μl) of bacterial suspension were incubated with 100 μl of PLL@AuNPs for 15 min at 37 °C under orbital shaking. Finally, the samples were washed twice with phosphate buffered saline (PBS) and resuspended in 1 ml of sterile double-distilled water.

SERS

SERS analyses were carried out using a Senterra II Raman instrument (Bruker, Optik GmbH, Germany), equipped with a 785-nm excitation laser and a × 50 objective. Prior to the measurements, 50 μl of each sample was deposited onto an aluminum foil by drop-casting. Bacteria were inactivated through a 5-min exposure to chloroform vapors within a glass chamber and dried under vacuum. The laser power was set at 10 mW, in order to avoid any sample destruction. Spectra were accumulated with a 5-s exposure time and 10 repetitions. To ensure the accuracy of the results, average spectra were obtained from three independent measurements collected under identical conditions.

Results and discussion

Figure 1a shows the UV-Vis absorption spectrum of the PLL@AuNPs, with an SPR peak at 532 nm, strongly indicating the presence of spherical nanoparticles with a narrow size distribution [18]. Importantly, UV-Vis analysis performed on different PLL@AuNPs batches generated similar spectra, indicating the reproducibility of the synthetic procedure. Also, the SPR profile was found to be unchanged over the 8-month period, thus demonstrating the high stability of the PLL-reduced nanogold. The XRD diffractogram (Fig. 1b) showed peaks for the (111), (200), (220), and (311) hkl planes at 38.2°, 44.3°, 64.6°, and 77.8°, respectively, with peak intensities corroborated with standard gold (Joint Committee on Powder Diffraction Standards-JCPDS card no 04-0784), in agreement with a face-centered cubic (FCC) crystalline nature. TEM was used to analyze the size and morphology of the construct. As suggested by SPR analysis, spherical particles with a narrow size distribution were observed (Fig. 1c). Also, negative staining of the samples indicated the presence of a thin semitransparent layer surrounding the particles [19], as expected for PLL-containing structures.

Fig. 1
figure 1

PLL@AuNPs structural features. a UV-Vis spectra show SPR, reproducibility, and stability of the particles (inset: photograph of an aqueous dispersion), b XRD pattern, c TEM image of negatively stained particles

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Table 1 FTIR peak assignment to functional groups of PLL@AuNPs
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The possible formation mechanism of PLL@AuNPs is due to acidic character and significant quantity of primary amines from PLL resulting in the reduction and stabilizing the nanoparticles [20]. The average hydrodynamic size distribution was measured using DLS, leading to the detection of 25.6-nm particles (Fig. 2a). In agreement with the presence of protonated amines from the PLL-decorated particles, a zeta potential analysis (Fig. 2b) showed a value of + 57 mV. FTIR analysis demonstrated the presence of absorption bands typifying the aminated polypeptide structure (Fig. 2c, Table 1) [20]. Altogether, these data suggested the suitability of our construct for tagging negatively charged surfaces through simple electrostatic interaction. We, therefore, evaluated whether PLL@AuNPs could adhere to microbial surfaces without tedious functionalization processes and transduce such interaction into optical changes. As a proof of concept, particles were incubated with E. coli viable cells for a 5-min period and washed, and optical spectroscopy data of the pelleted particles were obtained (see experimental details). As depicted in Fig. 3a, TEM analysis confirmed the attachment of PLL@AuNPs to E. coli cells, as expected. A UV-Vis analysis of the formed complexes (see experimental details) displayed a bathochromic shift from 532 to 550 nm, thus indicating the aggregation of nanoparticles onto bacterial cells (Fig. 3b). Finally, to explore the potential applicability of PLL@AuNPs as a microbial sensing tag, a SERS analysis of bacterial-particle complexes was performed. Mid-log E. coli cell cultures were extensively washed, incubated with PLL@AuNPs, and analyzed by Raman spectroscopy. Although there has been concerns regarding the contributions of residual growth media to the SERS signals of bacterial cultures, it has been demonstrated that properly washed cells do not generate SERS signals attributable to growth media components [21]. In this study, the exposure to PLL@AuNPs of E. coli cells led to enhanced “fingerprint” signals previously reported for E coli (Fig. 3c). In Table 2, the assignment of vibrational bands to functional/molecular identities is shown. Most of these signals correspond to the contribution of cell wall components, such as lipids or membrane proteins, or to DNA, typical of E. coli SERS spectra in the presence of different gold substrates [22, 23]. To note, no peaks featuring LB or yeast extract culture media, at ca. 735 cm−1, were found [21]. Therefore, these data support the use of PLL@AuNPs herein described as an easy-to-obtain nanotag for SERS-based detection of microbes (Fig. 4). Further research to study the application of these particles for monitoring microbial populations and for microbial diagnostics is encouraged. Likewise, the exploration of PLL@AuNPs to improve existing nanogold-based antimicrobial procedures, such as photodynamic inactivation [24] or antimicrobial treatments [25], merits further investigation.

Fig. 2
figure 2

Size, charge, and functional groups of PLL@AuNPs. a DLS hydrodynamic size distribution, b zeta potential, c FTIR spectra

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Fig. 3
figure 3

E. coli tagging by PLL@AuNPs. a TEM analysis, b UV-Vis spectra at different particle/cell ratios, c SERS spectra of bacterial cells, particles, and bacterial-particle complexes

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Table 2 Tentative SERS peak assignment to functional groups of E. coli compounds
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Fig. 4
figure 4

Tagging strategy of microbial cells using functional nanogold for SERS detection

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Conclusions

A facile method to synthesize optically active, cationic Au nanoparticles using a biocompatible polypeptide (PLL) using one-pot procedure was developed. The characterization techniques which showed a spherical nanogold of about 25 nm were obtained. The suitability of the construct PLL@AuNPs to attach negatively charged bacterial cells was confirmed by their ability to interact with viable E. coli cells upon a short incubation period. Moreover, the size of the obtained particles seemed suitable for the applicability of the construct as a label-free optical tool in microbial analysis. Further research to assess the potential of PLL@AuNPs in microbial sensing is therefore encouraged.