Abstract:
An identification and verification system and a process for said identification and verification of documents is disclosed, which is based on the use of nanoparticles embedded or adsorbed in the document support, utilising the different optical reflectance characteristics thereof in order to obtain, by combining several nanoparticles with specific characteristics, a high effectiveness in the identification of counterfeits.

Description:
OBJECT OF THE INVENTION 
       [0001]    The present invention relates to a system and a method for identifying documents or any another paper supports, such as paper money, equipment, consumption goods or other supports such as liquids. 
         [0002]    The object of the invention involves a system and a method for identifying and verifying a number of markers made up of embedded nanoparticles or nanoparticles that form a coating on the support. 
       BACKGROUND OF THE INVENTION 
       [0003]    Amongst the previous uses given to nanoparticles composed of a dielectric nucleus and a metal shell, different research groups worldwide have used them in medicine for the thermal ablation of tumour cells: they are injected in a tumour and a laser in the near-infrared region (NIR), with the wavelength whereat the particles are absorbed and diffracted, is externally applied; the particles are heated, resulting in the death of the tumour tissue due to temperature elevation, as reflected in patents US2002103517-A1 and US6530944-B2, and in several popular science documents. 
         [0004]    Some types of nanoparticles have also been patented as filling material for paints, to obtain thermal insulation paints, as disclosed in United States patent US6344272-B1 held by UNIV RICE WILLIAM MARSH. 
         [0005]    Also well-known are their plasmon optical resonance properties, which make them of interest to be used as contrast agents in medical imaging (by means of photoacoustic tomography), and their use is disclosed in various documents, such as patents US2002187347-A1 and US7144627-B2. They have also been used to activate an optically activated sensor in patents US2004214001-A1 and US7371457-B2. 
         [0006]    Other similar applications also consider the use of nanoparticles as optically activated valves; this use is reflected in documents such as  Optically controlled valves for microfluidics devices . Sershen, S. R., Ng, M. A., Halas, N. J., Beebe, D., West, J. L. Advanced Materials, 17 (2005): 1366-1368. 
         [0007]    Currently, there are other inorganic nanoparticles the use whereof is aimed at optical labelling. However, these nanoparticles are based on carbon (e.g. carbon nanotubes) or quantum dots (semiconductor nanostructures that confine the movement, in the three spatial directions, of conduction band electrons, valence band gaps or excitons (binding pairs of conduction band electrons and valence band gaps, CdSe, CdS, CdTe, etc.)) (e.g. ©Evident Technologies, Inc.). Said materials emit at a single wavelength within the near-infrared region. 
         [0008]    Invention patent US20070165209 discloses a method and a device for applying security labels or identifiers to documents or banknotes in order to prevent the counterfeiting thereof. Said identifiers may have the form of nanolabels, which may be Raman-active metal nanoparticles. 
         [0009]    More specifically, gold nanoparticles may heat an area of up to 1000 times their size when they are excited with a laser of a given wavelength. Said property has been used to produce the photothermal ablation of tumours in vitro and in vivo, as previously mentioned. These nanoparticles are formed by a dielectric nucleus (silica) and a shell made of gold or any other noble metal (i.e. silver, platinum, copper). By changing the relative dimensions between the materials that make up the nucleus and the shell, it is possible to modify the properties of the resonant plasmon (wavelength of optimal optical extinction) of gold, causing them to absorb light in the near-infrared region (NIR). This near-infrared region (between 800 and 1200 nm) is of interest in biomedical applications, since tissues are transparent in said region, and do not absorb light from the incident beam. It is the so-called “water window”. Thus, if a tissue is irradiated with any laser of a wavelength within that range, the temperature of said tissue will not rise. However, if the tissue is infiltrated with gold/silica nanoparticles, the application of a laser in the area would cause cell death by hyperthermia. Some authors have studied the effect of different nanoparticle geometries and shapes/thicknesses on the absorption of IR radiation, but always from the standpoint of biomedical use, in phototherapy and thermal ablation. 
       DESCRIPTION OF THE INVENTION 
       [0010]    A system is proposed for the authentication of various objects (identification documents, banknotes, paper money, luxury item labels, etc.), which is based on the use of nanoparticles that have a characteristic radiation absorption pattern in the near-infrared region (NIR). To this end, hybrid nanoparticles have been synthesised, composed of a dielectric nucleus made of silica and coated with a layer of gold, which present absorption patterns that may be modified as a function of the dimensions of the dielectric nucleus and the thickness of the metal layer. A given combination of dimensions provides a defined absorbance at a given wavelength (e.g. 808 nm) and at no other region of the spectrum. 
         [0011]    Moreover, in that region of the spectrum, called “water window”, few materials absorb light. I.e., below this region of the spectrum (between 800 and 1200 nm), light is absorbed by materials carrying chromophores, and, above this region, it is absorbed by materials containing water. For example, if we apply laser radiation of a wavelength within this region between 800 and 1200 nm to our skin, our skin and our bones would not absorb it and would be transparent thereto, as previously mentioned. This is obviously of great interest in medical applications, as discussed in the preceding section, and, for this reason, many works have attempted to develop this type of nanoparticles for various biomedical scenarios (cell labelling, hyperthermia, etc.), where the particles act as radiation targets. 
         [0012]    The application of the object of the invention is clearly different; in this invention, the nanoparticles composed of a dielectric nucleus and a metal shell are used to authenticate objects whereto the aforementioned nanoparticles have been incorporated, since said nanoparticles absorb in this NIR region and absorb exclusively at a given wavelength. The practical interest and the advantages thereof are evident, since these particles are highly sophisticated and their manufacturing is beyond the reach of most research laboratories, and, of course, counterfeiters; nonetheless, they may be manufactured at a low cost; given their nanometric size, they are invisible to the eye and even to optical microscopes; they do not change the essential properties of the material; they provide a means of authentication based on easy-to-read properties (light absorption at a given wavelength, supplemented, when applicable, with magnetic measurements); they may be easily introduced in both paper-based materials (e.g. cellulose, cotton, linen, etc.) and textile fibres and polymers; they may be dispersed in a liquid to be used as ink; unlike other nanoparticle-based systems, which usually work with fixed absorption characteristics for a given system, the system proposed in this invention presents high flexibility in its light absorption configuration, and an infinite number of absorbance patterns are possible, depending on the characteristics of the nanoparticles used. 
         [0013]    A first embodiment of the object of the invention proposes the use of a combination of these nanoparticles to obtain an optical label (and, if applicable, a magnetic label, if they are used combined with magnetic nanoparticles) which is characteristic and exclusive, in order to make it impossible to copy items that are labelled or embedded with said nanoparticles. This application is clearly different from those already known, since the nanoparticles composed of a dielectric nucleus and a metal shell disclosed in this invention are used to authenticate objects due to the fact that they absorb in this NIR region and absorb exclusively at a given wavelength. 
         [0014]    Another embodiment of the object of the invention also proposes the possible use of a combination of several types of nanoparticles to obtain an optical label (or, if applicable, a magnetic or a combined label) which is characteristic and exclusive, in order to make it practically impossible to copy items that are labelled or embedded with said nanoparticles; the ratio between the size of the nucleus and the size of the shell, to obtain optical tracks which are characteristic and exclusive, in order to prevent the counterfeiting not only of paper money, but also of equipment, high-value-added consumption goods, etc. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0015]    In order to complement the description being made and to contribute to a better understanding of the characteristics of the invention, in accordance with a preferred practical embodiment thereof, a set of drawings is attached to said description as an integral part thereof, wherein the following has been represented for illustrative, non-limiting purposes: 
           [0016]    FIG.  1 .—Shows a TEM image of the nanoparticles synthesised. 
           [0017]    FIG.  2 .—Shows an absorbance graph of silica/gold nanoparticles with a nucleus size of 50 nm and an absorbance maximum at 826 nm. 
           [0018]    FIG.  3 .—Shows an absorbance graph of silica/gold nanoparticles with a nucleus size of 100 nm and an absorbance maximum at 713 nm. 
       
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       [0019]    In light of the figures, below we describe a preferred embodiment of the process of this invention. 
         [0020]    For the embodiment of the object of the invention, two types of silica/gold nanoparticles were synthesised, with different relative sizes, in order to obtain different absorption properties of the resonant plasmon thereof. To this end, classic wet chemistry techniques are used to synthesise the materials. The sol-gel technique is used for the dielectric nucleus made of silica, by means of the Stöber method, and seeding and secondary growth is used to obtain the gold shell in accordance with the method described by Oldenburg et al. 
         [0021]    Thus, siliceous nanoparticles are obtained that are functionalised with amino groups in order to achieve the heterogeneous crystallisation, on the surface thereof, of gold particles (prepared separately) which grow to form layers of said material following successive regrowth steps with a gold precursor (chloroauric acid). 
         [0022]    Once obtained, the nanoparticles are characterised by means of: 
         [0023]    Transmission electron microscopy, to determine the size of the nanoparticles. The Dual Beam (Nova™ 200 NanoLab) equipment was used to distinguish the dielectric nucleus from the shell made of gold. 
         [0024]    High-resolution transmission electron microscopy, to perform electron diffraction and corroborate the crystalline nature of the gold shell surrounding the amorphous silica nucleus. To this end, HRTEM equipment from TEI Instruments was used. 
         [0025]    Nitrogen adsorption/desorption, to determine the specific surface area of the materials synthesised, by means of a Nitrogen adsorption equipment from Micromeritis. 
         [0026]    Photon correlation spectroscopy, to determine the hydrodynamic size of the nanoparticles in dispersion in different media and at different pHs, in a Malvern Zeta Sizer 2000 equipment. 
         [0027]    Thermogravimetry, to determine the quantity of amino groups on the surface prior to having the gold shell grow on the dielectric nuclei. 
         [0028]    Atomic Absorption and Emission Analytical Spectrometry (ICP), to determine the elementary composition of the materials. 
         [0029]    X-ray spectroscopy (XPS), to determine the atomic number of the elements that make up the surface of the nanoparticles, as well as the bonds found. 
         [0030]    Fourier Transform Infra-red Spectroscopy (FTIR) in a catalytic chamber (DRIFT), to determine the bonds and interactions between the materials, and the coatings and functionalisations thereof. 
         [0031]    UV-VIS-NIR spectroscopy, to evaluate the extinction coefficients and determine whether the nanoparticles synthesised absorb or disperse light within the near-infrared range, between 800 and 1200 nm. 
         [0032]    Study of the reproducibility of the synthesis and the stability of the nanoparticles formed through time, under standard storage conditions in the presence and in the absence of light. 
         [0033]      FIG. 1  shows the morphology of the nanoparticles synthesised. 
         [0034]      FIGS. 2 and 3  show how, by changing the proportions between the size of the nucleus and the size of the shell, absorption spectra in the near-infrared region are obtained which are characteristic of each nanoparticle.