An apparatus for measuring a concentration of a target molecule from a biological sample, the apparatus comprising: a housing including a top housing portion having a top opening and a bottom housing portion having a bottom opening, the top housing portion and bottom housing portion defining a cavity therebetween when attached to one another to form an assembled housing; a stack of material components disposed in said cavity adjacent to the top opening and the bottom opening such as to protrude outwardly from a side of the housing, the stack including: a lower waterproof membrane layer; a porous pad layer on top of the lower waterproof membrane layer, the porous pad layer comprising an unfilled capillary matrix and impregnated with a water-miscible solution containing an agent sensitive to the target molecule; a top hydrophobic and gas permeable membrane layer on the porous pad layer; wherein the lower waterproof membrane layer is coupled to the bottom housing portion to be visually and fluidly accessible through the bottom opening and the top membrane layer is coupled to the top housing portion to be visually and fluidly accessible through the top opening; and a biological sample-collecting device that includes: a tray defining a tray opening on a side of the tray, the tray opening to accommodate a portion of the stack of material components protruding from the side of the housing; a container defining a fluid passage therethrough operably attached on top of the tray; and a funnel cooperated with and fluidly leading through the container to the tray, the biological sample-collecting device configured to be cooperated with the assembled housing containing the stack of material components while receiving said portion of the stack of material components into the tray opening. a housing including a top housing portion having a top opening and a bottom housing portion having a bottom opening, the top housing portion and bottom housing portion defining a cavity therebetween when attached to one another to form an assembled housing; a stack of material components disposed in said cavity adjacent to the top opening and the bottom opening such as to protrude outwardly from a side of the housing, the stack including: a lower waterproof membrane layer; a porous pad layer on top of the lower waterproof membrane layer, the porous pad layer comprising an unfilled capillary matrix and impregnated with a water-miscible solution containing an agent sensitive to the target molecule; a top hydrophobic and gas permeable membrane layer on the porous pad layer; wherein the lower waterproof membrane layer is coupled to the bottom housing portion to be visually and fluidly accessible through the bottom opening and the top membrane layer is coupled to the top housing portion to be visually and fluidly accessible through the top opening; and a lower waterproof membrane layer; a porous pad layer on top of the lower waterproof membrane layer, the porous pad layer comprising an unfilled capillary matrix and impregnated with a water-miscible solution containing an agent sensitive to the target molecule; a top hydrophobic and gas permeable membrane layer on the porous pad layer; wherein the lower waterproof membrane layer is coupled to the bottom housing portion to be visually and fluidly accessible through the bottom opening and the top membrane layer is coupled to the top housing portion to be visually and fluidly accessible through the top opening; and a biological sample-collecting device that includes: a tray defining a tray opening on a side of the tray, the tray opening to accommodate a portion of the stack of material components protruding from the side of the housing; a container defining a fluid passage therethrough operably attached on top of the tray; and a funnel cooperated with and fluidly leading through the container to the tray, a tray defining a tray opening on a side of the tray, the tray opening to accommodate a portion of the stack of material components protruding from the side of the housing; a container defining a fluid passage therethrough operably attached on top of the tray; and a funnel cooperated with and fluidly leading through the container to the tray,