SERS of insecticides and fungicides assisted by Au and Ag nanostructures produced by laser techniques

The properties of noble metallic nanostructures (NSs) have been the subject of considerable fundamental and technological interest. Metal nanoparticles (NPs) play an important role in scientific investigation and nanotechnology. As a result of the progress in nanotechnologies during the last two decades, nanosystems find nowadays application in many areas, such as chemistry, optics, biology, agriculture, medicine, microelectronics, etc. [1].

The excitation spectrum of noble metallic sub-wavelength structures is determined by its surface plasmon resonance. The energy of the plasmon resonance depends strongly on the shape and composition of the nanostructures. The tunability of the plasmon resonances of noble metallic NPs can be exploited to position the optical resonances at specific wavelength regions of interest and has led to a wide range of applications. The strong local electro-magnetic field enhancement accompanying the surface plasmon resonances has also been used to manipulate light-matter interactions, so that noble metallic sub-wavelength structures have been widely applied in surface enhanced Raman spectroscopy (SERS) [2,3]. The enhancement of the Raman signal may reach a factor of 108-1012  the method is thus capable of detecting even a single molecule [2]. The increase of the Raman signal is a result of a local electromagnetic field enhancement in the vicinity of a structured surface due to the excitation of local and surface plasmons. In addition to their fundamental importance, plasmonic nanostructures are receiving a great deal of attention for their potential applications in areas such as sub-wavelength waveguides, optical nanoantennas, photovoltaic technology for efficient light coupling into solar cells, metamaterials, chemical and biological sensing, and biomedical applications [4-7].

Among the physical techniques, laser-based syntheses of nanomaterials have constituted a continuously growing field of research. In particular, pulsed laser ablation of solid targets in different environments, e.g. vacuum, background gas or liquid, has become an attractive method for the generation of NPs and deposition of NPs-assembled materials [8]. Among the successfully applied techniques one can mention the ns-laser deposition of thin metal films and the post-deposition structuring [9-13]. The nanostructuring of thin metal films by excimer laser pulses has been introduced as a technique for production of nanoparticles on different substrates [12,14]. The fragmentation of the metal surface into nanosized droplets during the melting is due to the poor wetting between the substrate and the liquid phase [15]. Moreover, femtosecond laser nanostructuring of silicon-based SERS substrates has also been reported [14,16].

Auand Ag NSs produced by laser methods have been used successfully in SERS analyses. Thus, an enhancement has been observed of the R6G Raman spectrum on Aunanocolumns formed by off-axis pulsed laser deposition (PLD). Concentrations of R6G as low as 1 nМ [3] have been measured and a maximum enhancement higher than 105 has been achieved. SERS has been used for trace analyses and detection of residue of different organophosphorous pesticides and insecticides [17-22] by employing various nanostructures, such as a solution of 100 nm Agnanocubes [18] and a Klarite Au-coated SERS-active substrate [19-21]. The SERS analyses have been compared with the traditional analyses, as chromatography, fluorescence polarization immunoassay, multi-enzyme inhibition assay, and biosensors. Although the traditional methods can be used to detect trace amounts of pesticides, they are time-consuming, labor-intensive and expensive, which makes them less attractive and limiting to a certain extent the laboratory, real-time, and field detection [19]. Among the large variety of pesticides, dimethoate is a widely-studied substance. The lowest concentration registered has been 5÷10 µg mL-1 using confocal Raman micro-spectrometry with Klarite substrates [19]. Generally, a low concentration at about 10-6 can be detected.

The Dithane DGfungicide (mancozeb as an active substance) and the Aktara 25 BGinsecticide (thiamethoxam as an active substance) are among the chemicals used extensively in agricultural medicine as protective substances for vegetables, fruits, crops etc. The fungicides from the dithiocarbamate group, as thiram, ferbam, ziram, metiram and zineb, have been widely studied in different SERS configurations using Au or Ag NSs [23-27]. Moreover, chemically produced gold nanorods have also been used as active SERS substrates for detection of ultra-low levels of fungicides from the dithiocarbamate group, i.e. thiram, ferbam and ziram [28]. Limits of detection as low as several nMhave been achieved depending on the type of material tested. Furthermore, recording of Fourier transform Raman and surface-enhanced Raman spectra of a silver colloid of the mancozeb dithiocarbamate fungicide has been reported [29]; a slight decomposition has been observed of this material because of a metal exchange with the silver on the surface. As for the insecticide Aktara 25 BG (thiamethoxam), to the best of our knowledge no SERS study has been reported.

The aim of this paper is to describe a study on and the results of the development of laser technologies for formation of advanced Agand Aunanostructures (NSs) by annealing PLD films on quartz. The as-produced NSs were used as active substrates for high-resolution analyses (SERS) to detect the Dithane DGfungicide and the Aktara 25 BGinsecticide. Two types of concentrations of the two materials were analyzed and compared  low, as in routine agricultural use, and high, as offered on the market. The SERS spectra of Auand Ag NPs-covered areas and films were compared with the Raman spectra taken from high concentrations deposited directly on glass.