**Ag-Nanostars for the Sensitive SERS Detection of Dyes in Artistic Cross-Sections—** *Madonna della Misericordia* **of the National Gallery of Parma: A Case Study**

### **Maria Sole Zala**ffi **<sup>1</sup> , Ines Agostinelli <sup>2</sup> , Najmeh Karimian <sup>1</sup> and Paolo Ugo 1,\***


Received: 29 September 2020; Accepted: 10 November 2020; Published: 12 November 2020

**Abstract:** In historical paintings, the detection of low amounts of pigments and dyes by Raman spectroscopy can sometimes be challenging, in particular for fluorescent dyes. This issue can be overcome by using SERS (surface-enhanced Raman spectroscopy) which takes advantage of the properties of nanostructured metal surfaces to quench fluorescence and enhance Raman signals. In this work, silver nanostars (AgNSs) are applied for the first time to real art samples, in particular to painting cross-sections, exploiting their effective SERS properties for pigment identification. The case study is the Madonna della Misericordia of the National Gallery of Parma (Italy). Cross-sections were analyzed at first by optical microscopy, SEM-EDS, and micro-Raman spectroscopy. Unfortunately, in some cross-sections, the application of conventional Raman spectroscopy was hindered by an intense background fluorescence. Therefore, AgNSs were deposited and used as SERS-active agent. The experimentation was successful, allowing us to identify a modern dye, namely copper phthalocyanine. This result, together with the detection of other modern pigments (titanium white) and expert visual examination, allowed to reconstruct the painting history, postdating its realization from the 15th century (according to the Gallery inventory) to 19th century with a heavy role of recent (middle 20th century) restoration interventions.

**Keywords:** silver nanostars; surface enhanced Raman spectroscopy; cross-sections; copper phthalocyanine; blue pigment; analytical diagnostics; National Gallery of Parma; Madonna della Misericordia
