Buoyant particulate strategy for few-to-single particle-based plasmonic enhanced nanosensors

Detecting matter at a single-molecule level is the ultimate target in many branches of study. Nanosensors based on plasmonics have garnered significant interest owing to their ultrahigh sensitivity even at single-molecule level. However, currently, plasmonic-enhanced nanosensors have not achieved excellent performances in practical applications and their detection at femtomolar or attomolar concentrations remains highly challenging. Here we show a plasmonic sensing strategy, called buoyant plasmonic-particulate-based few-to-single particle-nanosensors. Large-sized floating particles combined with a slippery surface may prevent the coffee-ring effect and enhance the spatial enrichment capability of the analyte in plasmonic sensitive sites via the aggregation and lifting effect. Dimer and single particle-nanosensors demonstrate an enhanced surface-enhanced Raman spectroscopy (SERS) and a high fluorescence sensitivity with an enrichment factor up to an order of ∼104 and the limit of detection of CV molecules down to femto- or attomolar levels. The current buoyant particulate strategy can be exploited in a wide range of plasmonic enhanced sensing applications for a cost-effective, simple, fast, flexible, and portable detection.


Supplementary Tables
Supplementary Table 1

Supplementary Notes
Supplementary Note 1: Calculation of solvent and molecules enrichment factor.
The enrichment efficiency of solvent and molecules on the hydrophobic slippery surface was evaluated based on the changes of solvent surface area during evaporation. Firstly, we estimated the surface area of the initial droplet with 60 μL in volume. As shown in Supplementary Fig. 18, the drop was ~5.8 mm in diameter, and thus the surface area S1 could be obtained as following: After the droplets drying on the hydrophobic slippery substrate, almost all solvent and molecules were concentrated on the surface of single particle or into the interface region of dimer particles. In case of single buoyant particle ( Supplementary Fig. 18), we assumed that all of solvent and probe molecules were concentrated on the surface region of single particle according to the experiment results. Thus, the surface area S2 could be obtained as following: In case of dimer buoyant particle, most solvent and analytes were concentrated into the gaps region according to the fluorescence imaging in Fig.2a. We assumed that all molecules were guided into the marked region shown in Fig. 2c and Supplementary Fig. 7. The surface area S3 could be calculated as following: Lastly, the enrichment factors of solvent and molecules for single particle and double particles were defined as ɛ1 and ɛ1, which were calculated by the specific value of surface area before and after evaporation. Therefore, we could obtain the final ɛ1 and ɛ2 as following: -29 -

Supplementary Note 2: Theoretical section
1. To understand the aggregation mechanism of buoyant particulates, we developed an analytical model to describe the influence of floating-particle configurations on droplet evaporation. The model is rooted in the force analysis of the suspended particulate at three phase (air-water-substrate) interfaces (Fig. 3c). For the floating-particle at the final evaporation stage as shown in Fig. 3c, a thin wetting film may be formed between the particle and substrate. The liquid bridge covering particles is formed and the height of liquid surface is close to the particle diameter. Thus the driving force acting on the particle can be written as 1 where θp is the contact angle between the liquid surface and particle surface, as shown in Fig. 3c, which is a natural parameter about the substrate, solvent, and atmosphere. For the current system, the values of θp and θR can be regarded as invariant. Therefore, the driving force acting on the particle is only affected by the particle radius.
In a CCA evaporation mode, it is reasonable to assume that the sliding friction force of the particle is approximately the friction force at the contact line, which is proportional to the value of (cosθRcosθe) from the unbalanced Young's stress. 2 -30 -From Eq.1 and 2, we note that the critical particle size is determined by θe, θp and θR. For the given system, θe, θp and θR are constant parameters. Hence, a critical particle size, Rc exists. When the size of floating-particle exceeds this value, the liquid capillary attraction Fp may dominate during the aggregating process.
where r' = ∂r/∂x, r" = ∂ 2 r/∂x 2 . k = Δp/(2σ), is the characteristic parameter for the shape of the pendular ring, where Δp is the pressure difference across the gas-liquid interface.
The pendular ring is constrained on the particle surfaces with two boundary conditions: where θp is the contact angle, β the filled-angle between the x-axis and the line contacting the The volume of the residual pendular ring is given by Once the shape of the pendular ring is determined for a certain particle radius R, the filledangle β, surface tension σ, relative volume of liquid with respect to the solid sphere volume, and the capillary force arising from the interaction between the solid and liquid surface can be obtained.
The capillary force comprises two parts: the surface tension term acting at the wetting perimeter, tangent to the meniscus at the intersection with the solid surface; the pressure difference term across the curved gas-liquid interface, which is computed over the axially projected wetting area of each particle.
-32 - Obviously, with a given filled-angle β, the wetting force (first item) is proportional to the particle radius. The unknown parameter in the second item is kc0. From the above equations, kc0 = kRsinβsin(α+β)+(kRsinβ) 2 , and the decisive parameter is kR. However, kR is constant at a given filled-angle. Under a given filled-angle β, the profile of the liquid ring is coaxial parallel at various radii. Considering the pressure difference, kR = (1/r1+1/r2)R/2, where r1 and r2 are two principle curvature radii that are both proportional to the particle radius with the coaxial parallel profile. Then, the Laplace force (second item) is also proportional to the particle radius with a given filled-angle β.
where K(β) = kR is the function of the filled-angle β. From Eq. (14), Fr increases with the particle radius for a certain filled-angle. Therefore, the liquid will be promoted to rise under vertical capillary force with a larger particle.
However, the vertical capillary force reduces along with the particle radius decrease. The appearance of liquid lifting is conditional for the micron scale particle. The vertical capillary force should be larger than zero for lifting the solvent liquid. Then, the critical radius for zero-Fr is