## 1 Introduction  
The interaction of organic molecules with metal nanoparticles (NPs) is of central importance for a large number of applications in several fields, such as chemical and biological sensing [1, 2, 3, 4], optics [5, 6, 7], energy [8, 9, 10, 11], catalysis [12, 13, 14, 15], and nanomedicine [16, 17, 18, 19]. Many of these applications are based on the effect of the localized surface plasmon resonance (LSPR) on the absorption, emission, and scattering of molecules attached to or in close vicinity to the NP surface [20, 21, 22, 23]. NPs have the remarkable property of strongly enhancing the local electromagnetic near-field [24, 25, 26]. This, together with the effect of the LSPR, can lead to the light absorption and scattering of NPs to be extremely strong, e.g., their absorption cross-sections are up to five to six orders of magnitude higher than the most strongly absorbing organic dye molecules. This has led to the use of NPs as beacons to enhance processes in nearby molecules [27, 28].  
The interaction of fluorophores with Au and Ag NPs is slightly more complicated, and both enhancement and quenching of fluorophores have been reported [29; 30; 31; 32; 33]. Part of the reason for this apparent contradiction is that the interaction between fluorophores and the NP depends on the size and shape of the NPs, the distance of the fluorophore from the surface of the NPs, the orientation of the fluorophore dipole moment, and the overlap between the emission spectrum of the fluorophore and the plasmon resonance spectral feature of the NP [34; 35; 36].  
The effect of dye quenching has been widely exploited for chemical and biological sensing applications, with one of the main techniques being fluorescence resonance energy transfer (FRET), in which the initially excited chromophore transfers its energy to an acceptor, thus quenching the fluorescence in the process [23; 37; 38]. In FRET, the donor and acceptor are molecules, and the energy transfer occurs via through space dipole-dipole interactions [39; 40; 41]. When a NP is used as an acceptor, the quenching is amplified up to 100 times, making the technique more sensitive. An elegant example of this technique is the use of single-stranded DNA (ssDNA) linked to a NP at one end and to a fluorophore at the other end. In the absence of a target DNA, the ssDNA is relaxed and the fluorophore is close to the surface of the NP, thus quenched through FRET. Only when the ssDNA is conjugated with the target DNA, the fluorophore moves away from the surface of the NP and the fluorescence is allowed to light up [42]. Based on this mechanism, a simultaneous recognition of ssDNA and double-stranded DNA (dsDNA) was also studied [43]. Biosensing often relies on fluorescence because this method is simple but sensitive. In some cases, nanomaterials can be an alternatives to traditional organic dyes or sometimes act as a solid support by simplifying the sensor design, increasing the labeling ratio for greater sensitivity and facilitating eVental recovery [44; 45; 46; 47; 48; 49].  
Among others, silver engineered nanomaterials are used in biosensing and imaging when conjugated with dyes [50; 51; 52]. Specially fluorescein isothiocyanate (FITC) and rhodamine B isothiamate (FITC) are widely used in sensing and as labeling systems [53; 54; 55; 56]. To fully exploit the potential of AgNPs conjugated with fluorescent molecules, it is necessary to have a complete understanding of the details of the structures and the interaction between the fluorophore blocked in a position very close to the NP.  
This paper focuses on the synthesis of AgNPs capped by FITC or RITC (i.e., AgNPs-FITC and AgNPs-RTTC). The deep characterizations of these nanosystems allowed us to clarify the chemical structure and eValuate the optical properties due to the direct interaction between plasmonic nanoparticles and fluorescent dyes.