## 1 Introduction  
Halloysite is an aluminosilicate material, which is naturally structured into eco-friendly, non-toxic nanosized rolls [1; 2]. The inner and outer surfaces of halloysite nanotubes (HNTs) have different surface charges [3] due to which HNTs have a wide range of applications as nanocontainers for both drug delivery [4] and other useful agents [5], and as matrices for the deposition of nanoparticles (NPs) [6]. HNT-based drug formulations demonstrate lower drug side effects, render the protection of drug molecules from possible degradation in aggressive conditions (low/high pH, enzymatic activity), increase the aqueous solubility of hydrophobic insoluble drugs, accumulate in pathological sites in the body, and help controlling drug release rates [7; 8; 9]. For example, a prodigiosin-HNT-based nanoformulation (p-HNTs) and its effects on the viability of malignant and non-malignant cells have been demonstrated [10]. NPs of plasmonic metals attached at the HNT's surface can form hot spots between each other, where local electric fields from each pair of NPs are enhanced [11; 12]. Moreover, plasmonic NPs, which are fixed on HNTs, are protected from sticking together, they form agglomerates less often, and retain their properties longer [11]. Note that the physical properties of nanostructures depend on their topology-driven interface conditions, and an example of the terahertz light absorption in semiconductor nanoshells resonantly enhanced due to the strong coupling between interface plasmons and phonons is demonstrated [13].  
Silver nanoparticles (Ag-NPs) are widely used as a nanomaterial with superior plasmonic properties, and the SERS activity of Ag-NPs is shown and developed in a variety of studies [14; 15; 16; 17; 18; 19]. The SERS phenomenon can also allow using such NPs in the solar energy conversion systems [20] and as biosensors for the detection of different substances both in water and in vitro [17; 21]. Ag-NPs deposited on the HNTs surface can result in localized surface plasmon resonance (LSPR) in the region of 400 to 600 nm [11; 22] and thus it could be used for SERS [3]. It was shown that a noticeable SERS response is observed upon the addition of Rhodamine 6G dye to the HNTs:Ag-NPs nanocomposite [11]. Note, the SERS activity of HNTs:Ag-NPs persists even after soaking in aqueous solution for a week [11]. While Ag-NPs have a wide range of applications in disinfection [14], the possibility of their deposition on the HNT's matrix makes it possible to enhance the effect by combining the bactericidal action of silver ions with electric field amplification from LSPR modes [22].  
Gold nanoparticles (Au-NPs) are similarly well-known for their SERS ability [23; 24]. Nanocomposites based on HNTs and Au-NPs are often explored for catalytic applications [25], including precisely due to the plasmonic properties of Au-NPs [26]. Herewith, the protocol for \"growing\" Au-NPs on the surface of the HNTs is well-developed, since the efficiency of HNTs as SERS substrate has been proven [27]. While the SERS of Rhodamine 6G (R6G) at the concentration of \(10^{-3}\) M was found for HNTs:Au-NPs, the reported rather low sensitivity and required large sizes on Au-NPs of about 50 nm can be limiting factors for further applications [27].  
We report on the SERS activity of HNTs decorated with surface-immobilized Ag- and Au-NPs with men sizes from 5 to 10 nm. The plasmonic properties of HNTs:Au/Ag-NPs nanocomposites are also simulated to reveal the role of the NPs' geometric characteristics: size distribution and spatial localization on the nanotube' surface. The potential of the prepared nanocomposites for the SERS analysis of biosystems is demonstrated by experiments in vitro.