RESEARCH PAPER Development of uncoated near-spherical gold nanoparticles for the label-free quantification of Lactobacillus rhamnosus GG by surface-enhanced Raman spectroscopy Elie Akanny 1,2 & Anne Bonhommé 1 & Carine Commun 3 & Anne Doleans-Jordheim 3,4 & François Bessueille 1 & Sandrine Bourgeois 2,5 & Claire Bordes 2 Received: 22 March 2019 /Revised: 1 May 2019 /Accepted: 22 May 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The Surface-enhanced Raman spectroscopy (SERS) method based on gold nanoparticles as SERS substrate was investigated for the label-free detection and quantification of probiotic bacteria that are widely used in various pharmaceutical formulations. Indeed, the development of a simple and fast SERS method dedicated to the quantification of bacteria should be very useful for the characterization of such formulations in a more convenient way than the usually performed tedious and time-consuming conventional counting method. For this purpose, uncoated near-spherical gold nanoparticles were developed at room temperature by acidic treatment of star-like gold nanoparticle precursors. In this study, we first investigated the influence of acidic treatment conditions on both the nanoparticle physicochemical properties and SERS efficiency using Rhodamine 6G (R6G) as Bmodel^ analyte. Results highlighted that an effective R6G Raman signal enhancement was obtained by promoting chemical effect through R6G-anion interactions and by obtaining a suitable aggregation state of the nanoparticles. Depending on the nanoparticle synthesis conditions, R6G SERS signals were up to 10 2 –10 3 -fold greater than those obtained with star-like gold nanoparticles. The synthesized spherical gold nanoparticles were then successfully applied for the detection and quantification of Lactobacillus rhamnosus GG (LGG). In that case, the signal enhancement was especially due to the combination of anion-induced chemical enhancement and nanoparticle aggregation on LGG cell wall consecutive to non-specific interactions. Both the simplicity and speed of the procedure, achieved under 30 min, including nanoparticle synthesis, sample preparation, and acquisition of SERS spectra, appeared as very relevant for the characterization of pharmaceutical formulations incorporating probiotics. Keywords Surface-enhanced Raman spectroscopy . Lactobacillus rhamnosus . Rhodamine 6G . Gold nanoparticles Introduction The identification and quantification of microorganisms are an essential issue in food and water safety, biodefense, human health care, and diagnostics [1]. The conventional counting method, using plating and culturing, is the most widely used method for microorganism quantification. But this method appears to be tedious and time consuming. The surface- Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00216-019-01938-4) contains supplementary material, which is available to authorized users. * Elie Akanny elie.kouame-akanny@univ-lyon1.fr 1 Institut des Sciences Analytiques, UMR 5280, CNRS, ENS Lyon, Université Lyon 1, Université de Lyon, 5 rue de la Doua, 69100 Villeurbanne, France 2 CNRS, LAGEPP UMR 5007, Univ Lyon, Université Claude Bernard Lyon 1, 43 boulevard du 11 novembre 1918, 69100 Villeurbanne, France 3 Equipe de Recherche Bactéries Pathogènes Opportunistes et Environnement, UMR CNRS 5557 Ecologie Microbienne, Université de Lyon 1 & VetAgro Sup, 69622 Villeurbanne, France 4 Laboratoire de Bactériologie, Institut des Agents Infectieux, Centre de Biologie et Pathologie Nord, Hospices Civils de Lyon (HCL), 69003 Lyon, France 5 ISPB-School of Pharmacy, Université Lyon 1, Université de Lyon, 69008 Lyon, France Analytical and Bioanalytical Chemistry https://doi.org/10.1007/s00216-019-01938-4