Periodontitis represents the most frequent cause of tooth loss in adults in industrialized countries, with potential significant consequences for masticatory function, aesthetics and also systemic health [1]. Recent studies have showed that periodontitis and systemic health are mutually correlated [2,3]. Particularly, periodontitis has been correlated with low-grade inflammation (LGI), a systemic status of chronic sub-clinical production of pro-inflammatory agents which represents a sneaky risk factor for many diseases such as diabetes, cardiovascular, cerebrovascular, neurodegenerative diseases, and even cancer [4,5].

There is evidence of a strong correlation between the onset or exacerbation of periodontitis and the presence in the gingival crevicular fluid (GCF) and, through this, in the saliva, of specific biomolecules correlated to an inflammatory status [6], [7], [8]. Currently, the diagnosis of periodontitis is predominantly clinical and radiographic, implying disease detection only after the occurring of the biological damage, with evident limits in terms of precision and ability to identify incipient forms or evaluate disease activity and risk of future progression. In this sense, the use of salivary biomarkers has been proposed as a useful aid to periodontal diagnosis, even allowing to identify the early stage of periodontitis, before the clinical and radiographic signs of tissue damage.

Matrix metalloproteinase-8 (MMP-8), also known as collagenase-2 or neutrophil collagenase, is one of the most documented candidates for the discrimination between periodontal health and disease [7,[9], [10], [11]]. MMP-8 plays a key role in degradation of both periodontal soft and hard tissues occurring during physiological remodeling, as well as under irreversible pathologic breakdown [12,13]. A minimal level of this collagenase exerts physiologic functions including (but not limited to) the processing of growth factors and protective endogenous proteinase inhibitors, and has been shown to exhibit anti-inflammatory properties [14], [15], [16]. MMP-8 responsible of matrix degradation and attachment loss in course of periodontitis has been demonstrated to be primarily derived from polymorphonuclear leukocytes (PMNs) of the diseased periodontium [17,18]. Subsequent to PMN degranulation, latent proMMP-8 transforms into its activated counterpart, as a result of reactive oxygen species interaction and/or proteolytic cleavage also determined by periodontopathogens-derived proteases [19], [20], [21], [22]. Several studies have investigated the important role played by active MMP-8 (aMMP-8) in the progression of periodontitis and suggested that elevated levels of aMMP-8, rather than total or latent MMP-8, may differentiate periodontitis from gingivitis, and precede periodontal attachment loss [13,[22], [23], [24], [25], [26]]. Therefore, MMP-8, especially in its active form, is considered one of the strongest markers of periodontitis with specificity ranging from 48 to 87% and sensitivity from 65 to 87% [[9], [27], [28], [29]].

Salivary biomarkers, including MMP-8, are primarily analysed via laboratory analyses such as the enzyme-linked immunosorbent assay (ELISA) or the immunofluorometric assay (IFMA). This kind of analyses, however, is expensive and requires long time to obtain a response. Furthermore, the currently available equipment is bulky and not portable. Alternatively, the development of small size biosensor analysis systems, namely point-of-care tests (POCTs), able to realize immediate and low-cost analysis directly at the patient care site, allow to monitor the rapid change of periodontal health conditions contributing to the immediate formulation of the correct diagnosis and treatment plan.

Different technologies have been proposed for the chair-side detection of MMP-8, such as the commercially available dip-stick immunoassay for aMMP-8 (PerioSafe®, Dentognostics GmbH), or the experimental total MMP-8 POCT based on the surface acoustic wave (SAW) technology [30].

Surface plasmon resonance (SPR) is a physical phenomenon at the basis of several devices used to measure the binding of an analyte with its specific receptor layer, usually over a planar metal (typically gold or silver) surface, by exploiting the refractive index variations at the interface between the metallic surface and the dielectric medium. SPR is widely used as a detection principle for different biosensors, in which the metallic surface is functionalized by immobilizing specific bioreceptors (i.e. recombinant antibodies, aptamers, etc.) producing a highly sensitive self-assembled monolayer (SAM). Recently, plastic optical fibers (POFs), suitably modified and functionalized with a gold nano-film covered by specific SAMs, have been proposed to produce SPR-based biosensors with ultra-low detection limits for a heterogeneous panel of analytes in different application fields [31].

The aim of this proof-of-concept study is to develop a novel highly sensitive POCT based on a POF biosensor exploiting SPR to detect salivary MMP-8.