## 2 Materials and Methods  
Streptavidin coated superparamagnetic beads of 1 um in diameter were purchased from Chemicell (Berlin, Germany). Gold-coated slides, phosphate buffered saline (PBS) and Bovine Serum Albumin were purchased from Sigma-Aldrich (St. Louis, MO, USA). Biotin-coupled bovine serum albumin (BBSA) was purchased from Thermo-Fisher (Waltham, MA, USA). Optical micrographs were recorded using Nikon (Melville, NY, USA) microscope equipped with a charge-coupled (CCD) camera, spherical lens was purchased from BioSpec (Bartlesville, OK, USA), PMMA was purchased from Mc Master Carr  
Figure 1: Illustration of the micro-contact printing on gold slides, self-assembled immunomagnetic beads on the printed receptors and cell phone imaging.  
Figure 2: Cell phone microscope using a spherical ball lens. (**A**) Illustration of the important parameters of the spherical ball lens; (**B**) Image of the cell phone microscope; (**C**) Schematic of the measurement.  
(Elmhurst, IL, USA). PMMA was cut with a laser cutter as a lens holder and attached to the Samsung Note S4 cell phone with double sided tape (Figure 2).  
The PDMS stamp that contains a 20 \(\upmu\)m of alternating lines was first cleaned; washed in ultrasonic bath with 70% ethanol and rinsed with 70% ethanol followed by DI water. Later stream of nitrogen gas was used to gently dry the stamp. The gold chips of size 8 \(\times\) 4 mm were prepared by first cutting the gold-coated glass slides, cleaning with ethanol and DI water followed by drying with nitrogen gas. The PDMS stamp was inked with 2 mg/mL of BBSA and BBSA solution was left on the stamp for 15 min before removing the excess solution using a wick of a tissue paper from the stamp. The stamp was again dried using a gentle stream of nitrogen gas. Then the stamp was brought into contact with the gold chip and a small force was applied to make a contact between the two surfaces. After waiting for 1 min, the stamp was removed and the gold chip was rinsed gently with PBS to remove the unbound BBSA. To passivate the rest of the surface that had no BBSA, the gold chips were covered a solution of BSA (1 mg/mL) and incubated for 2 min before washing with DI water to remove the unbound BSA.  
The gold chip containing the BBSA pattern was incubated with various concentrations (0.0335 mg/mL, 0.067 mg/mL, 0.67 mg/mL and 6.7 mg/mL) of streptavidin coated magnetic beads. After 20 min of waiting, the gold chip was washed with DI water and dried. Then the images of gold chips with immobilized streptavidin coated magnetic beads were recorded using the Nikon optical microscope and the cell phone microscopy (Figure 2).  
The Samsung Note 4 cell phone is equipped with a 16-megapixel (5312 \(\times\) 2988) camera composed of a Sony IMX240 imaging sensor and an \(f\)/2.2 lens with a 35 mm equivalent focal length of 31 mm.  
The spherical ball lens has a diameter of \(D\) = 3.51 mm and it is made of borosilicate having a refraction index of \(n=1.517\). The important parameters of the spherical ball lens were calculated using the formulas in Table 1, a is optimal aperture radius as explained in [19]. Another cell phone's flashlight was used as a light source, and a simple flashlight can be also considered as the light source.  
Nikon optical microscope with 10\(\times\) objective and 10\(\times\) eyepiece provides 100\(\times\) magnification and used as a reference imaging system to compare with cell phone microscope.