Applied Surface Science 388 (2016) 704–709 Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: www.elsevier.com/locate/apsusc MnO 2 -protected silver nanoparticles: New electromagnetic nanoresonators for Raman analysis of surfaces in basis environment Heman Burhanalden Abdulrahman, Karol Koł˛ ataj, Paweł Lenczewski, Jan Krajczewski, Andrzej Kudelski ∗ Department of Chemistry, Warsaw University, ul. Pasteura 1, Pl-02-093 Warsaw, Poland a r t i c l e i n f o Article history: Received 19 December 2015 Received in revised form 24 January 2016 Accepted 28 January 2016 Available online 1 February 2016 Keywords: Shell-isolated nanoparticle-enhanced Raman spectroscopy SHINERS Surface-enhanced Raman spectroscopy SERS Ag@MnO2 MnO2 sponge nanostructures a b s t r a c t The first example of the synthesis of Ag nanoparticles protected by a few nanometers thick layer of MnO 2 (Ag@MnO 2 ) has been reported. Synthesized Ag@MnO 2 nanoparticles effectively locally enhance the electric field of the incident visible radiation, which allows, for example, for a large enhancement of the efficiency of Raman scattering for species located in the close proximity to such nanostructures. It means that Ag@MnO 2 nanoparticles may be used as nanoresonators for shell-isolated nanoparticle-enhanced Raman scattering (SHINERS) measurements. The obtained Ag@MnO 2 nanoparticles are almost two orders of magnitude more efficient in enhancing Raman signal than previously used for SHINERS measurements in the alkali environment Au@MnO 2 nanostructures. Moreover, in comparison to Ag@SiO 2 nanoparticles, which are standard silver nanoresonators for SHINERS experiments, Ag@MnO 2 nanoparticles are signifi- cantly more stable in the basic conditions. Deposition of the MnO 2 layer (by the reduction of KMnO 4 with by K 2 C 2 O 4 in an alkaline condition) on hollow silver nanoparticles (h-Ag) has been also analyzed. Hol- low silver shells are significantly less stable than the solid Ag nanostructures and are practically entirely destroyed during the process of the MnO 2 deposition. However, in this condition, the majority of h-Ag nanoparticles form agglomerates containing about 10 1 h-Ag items which are connected by MnO 2 , and after dissolution of the silver auxiliary templates very regular MnO 2 sponge nanostructures with the diameter of 150–300 nm are formed. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Analysis of various surfaces is very important from the eco- nomic and scientific points of view. Very promising method for carrying out such analysis is developed by Tian et al. so-called shell-isolated nanoparticle-enhanced Raman spectroscopy (SHIN- ERS) [1]. In this method the investigated surface is covered by the layer of the plasmonic metal nanoparticles (Au, Ag) protected by very thin (typically 2–5 nm) layer of chemically inert dielectric, which should be also very weak Raman scatterer (SiO 2 , Al 2 O 3 or MnO 2 ) [1–5]. Nanoparticles of plasmonic metals act as electromag- netic resonators, significantly locally enhancing the electric field of the incident radiation, and hence leading to a large increase of the efficiency of Raman scattering from the molecules being in the close proximity to such plasmonic nanoparticles. Protecting dielec- tric layer separates metal nanoparticles from direct contact with the surface of the analyzed sample and keeps metal nanoparticles ∗ Corresponding author. E-mail address: akudel@chem.uw.edu.pl (A. Kudelski). from agglomerating. If the protecting dielectric layer is very thin (below 5 nm) it does not significantly damp the enhancement of the electromagnetic field [1]. Because SHINERS analysis may be car- ried out on the surface of the solid sample submerged in the high pressure gas or the liquid, SHINERS is a very promising technique for analysis of interfaces of biological objects in situ, which is very difficult with the majority of standard surface techniques [1–5]. SiO 2 and Al 2 O 3 , which are typically used for covering of plas- monic metal nanoparticles for SHINERS applications, could not be unfortunately used as protecting layer in the alkaline media [2]. For SHINERS measurements in this conditions Tian et al. sug- gested using gold nanoresonators protected by the MnO 2 layer (Au@MnO 2 ). In 2012 it was shown that using silver nanoparti- cles as SHINERS nanoresonators instead of the gold nanostructures allows for larger (even by two orders of magnitude) increase of the sensitivity of SHINERS analysis [6,7]. Therefore, in this work, we decided to synthesize MnO 2 -protected silver nanoresonators. We also tested deposition of manganese(IV) oxide on hollow silver (h-Ag) nanoparticles. Hollow silver nanoparticles exhibit surface plasmonic properties different than analogous solid structures. For example, the position of the extinction band in the vis spectrum of http://dx.doi.org/10.1016/j.apsusc.2016.01.262 0169-4332/© 2016 Elsevier B.V. All rights reserved.